Method and apparatus for performing handover in wireless communication system

ABSTRACT

A user equipment (UE) and method thereof are provided for use in a wireless communication system. The method includes receiving, from a source cell, a radio resource control (RRC) reconfiguration message; configuring a medium access control (MAC) entity for a target cell in response to the RRC reconfiguration message configuring a bearer to a dual active protocol stack (DAPS) handover; performing, by the MAC entity, a random access procedure on the target cell for the DAPS handover; transmitting, from the MAC entity, to an upper layer, an indication of completion of the random access procedure, in response to completing the random access procedure to the target cell; and switching, by a packet data convergence protocol (PDCP) entity, uplink data for the bearer, in response to receiving a request for uplink data switching from the upper layer based on the indication.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2019-0122653, filed in the Korean Intellectual Property Office on Oct. 2, 2019, and Korean Patent Application No. 10-2019-0138774, filed in the Korean Intellectual Property Office on Nov. 1, 2019, the entire disclosure of each of which is incorporated herein by reference.

BACKGROUND 1. Field

The disclosure relates generally to performing a handover in a wireless communication system, and more particularly, to a method and apparatus for efficiently performing a handover without interruption to data transmission and reception during the handover.

2. Description of the Related Art

To meet the increasing demand with respect to wireless data traffic since the commercialization of 4^(th) generation (4G) communication systems, efforts have been made to develop improved 5^(th) generation (5G) communication systems or pre-5G communication systems. 5G communication systems or pre-5G communication systems may be referred to as beyond 4G network communication systems or post long term evolution (LTE) systems.

To achieve a high data transmission rate, consideration is given to implementing 5G communication systems in millimeter wave (mmW) frequency bands (e.g., 60 GHz bands). In order to reduce propagation path loss of radio waves and increase a propagation distance of radio waves in the millimeter wave frequency bands, in 5G communication systems, discussions are underway about technologies such as beam-forming, massive multiple input multiple output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, analog beam-forming, and large-scale antennas.

To improve networks of systems, in 5G communication systems, development of technologies such as evolved small cell, advanced small cell, cloud radio access network (RAN), ultra-dense network, device to device communication (D2D), wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), and interference cancellation are underway. Further, for 5G communication systems, developments of an advanced coding modulation (ACM) scheme such as hybrid frequency-shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC) and an enhanced network access scheme such as filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) are underway.

The Internet is evolving to an Internet of things (IoT) network in which distributed components, i.e., things, transmit or receive information to or from each other and process the information. Internet of everything (IoE) technology in which big data processing technology is combined with IoT technology via connection with a cloud server, etc., is also emerging. To implement IoT, technical elements, such as sensing technology, a wired/wireless communication and network infrastructure, service interface technology, and security technology, are required, and thus a sensor network, machine to machine (M2M) communication, machine type communication (MTC), etc., for connection between things have recently been studied. In IoT environments, an intelligent Internet technology service for collecting and analyzing data generated by connected things may be provided. IoT is applicable to various fields, such as smart homes, smart buildings, smart cities, smart cars or connected cars, smart grids, health care, smart home appliances, and advanced medical care, via fusion and combination of existing information technology (IT) with various industries.

Accordingly, various attempts are being made to apply 5G communication systems to IoT networks. For example, technology such as a sensor network, M2M communication, or MTC is implemented by 5G communication technology such as beam-forming, MIMO, or array antenna. The application of a cloud RAN as big data processing technology may also be considered as an example of convergence of 5G technology and IoT technology.

A method of providing a plurality of connections has been proposed as one of many techniques for satisfying the increasing demand for high-capacity communication. For example, in carrier aggregation (CA) of an LTE system, a plurality of connections can be provided through a plurality of carrier waves. Accordingly, a user may be provided services through more resources than before. In addition, various services including a broadcasting service such as multimedia broadcast multicast services (MBMS) may be provided through the LTE system.

SUMMARY

The disclosure has been made to address the above-mentioned problems and disadvantages, and to provide at least the advantages described below.

An aspect of the disclosure is to provide an efficient handover method for supporting low transmission latency and a service without data interruption in a wireless communication system.

In accordance with an aspect of the disclosure, a method is provided for a user equipment (UE) in a wireless communication system. The method includes receiving, from a source cell, a radio resource control (RRC) reconfiguration message; configuring a medium access control (MAC) entity for a target cell in response to the RRC reconfiguration message configuring a bearer to a dual active protocol stack (DAPS) handover; performing, by the MAC entity, a random access procedure on the target cell for the DAPS handover; transmitting, from the MAC entity, to an upper layer, an indication of completion of the random access procedure, in response to completing the random access procedure to the target cell; and switching, by a packet data convergence protocol (PDCP) entity, uplink data for the bearer, in response to receiving a request for uplink data switching from the upper layer based on the indication.

In accordance with another aspect of the disclosure, a UE is provided for use in a wireless communication system. The UE includes a transceiver; and a processor coupled with the transceiver and configured to control the transceiver to receive, from a source cell, a radio resource control (RRC) reconfiguration message, configure a medium access control (MAC) entity for a target cell, in response to the RRC reconfiguration message configuring a bearer to a dual active protocol stack (DAPS) handover, control, the MAC entity, to perform a random access procedure on the target cell for the DAPS handover, control the transceiver to transmit, from the MAC entity, to an upper layer, an indication of completion of the random access procedure, in response to completing the random access procedure, and control, a packet data convergence protocol (PDCP) entity, to switch uplink data for the bearer, in response to receiving a request for uplink data switching from the upper layer based on the indication.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1A illustrates an LTE system, according to an embodiment;

FIG. 1B illustrates a radio protocol architecture in an LTE system, according to an embodiment;

FIG. 1C illustrates a next-generation mobile communication system, according to an embodiment;

FIG. 1D illustrates a radio protocol architecture of a next-generation mobile communication system, according to an embodiment;

FIG. 1E is a signal flow diagram illustrating a procedure in which a user equipment (UE) switches from a Radio Resource Control (RRC) idle mode to an RRC connected mode and configures a connection with a network, according to an embodiment;

FIG. 1F is a signal flow diagram illustrating signaling procedures for performing a handover in a next-generation wireless communication system, according to an embodiment;

FIG. 1G illustrates an efficient handover method for minimizing a data interruption time due to a handover, according to an embodiment;

FIG. 1H illustrates an efficient handover method for minimizing a data interruption time due to a handover, according to an embodiment;

FIG. 1I illustrates an efficient Packet Data Convergence Protocol (PDCP) layer for use in a Dual Active Protocol Stack (DAPS) handover method according to an embodiment;

FIG. 1J is a flowchart illustrating operations of a UE, according to an embodiment;

FIG. 1K illustrates a UE according to an embodiment; and

FIG. 1L illustrates a base station (BS) in a wireless communication system, to which an embodiment is applicable.

DETAILED DESCRIPTION

Operation principles of the disclosure will now be described more fully with reference to the accompanying drawings. In the disclosure, detailed descriptions of related well-known functions or configurations may be omitted when they may unnecessarily obscure the essence of the disclosure. Also, terms used below are defined in consideration of functions in the disclosure, and may have different meanings according to an intention of a user or operator, customs, etc. Thus, the terms of the disclosure should be defined based on the description throughout the specification.

Throughout the disclosure, the expression “at least one of a, b or c” indicates only a, only b, only c, both a and b, both a and c, both b and c, all of a, b, and c, or variations thereof.

Examples of a terminal may include a UE, a mobile station (MS), a cellular phone, a smartphone, a computer, a multimedia system capable of performing a communication function, etc.

In the disclosure, a controller may also be referred to as a processor.

A layer (or a layer apparatus) may also be referred to as an entity.

In the following description, terms identifying access nodes, terms indicating network entities, terms indicating messages, terms indicating interfaces between network entities, terms indicating various types of identification information, etc. are exemplified for convenience of explanation. Accordingly, the disclosure is not limited to terms used below, and other terms representing objects having the equivalent technical meaning may be used.

For convenience of description, the disclosure uses terms and names defined in the 3^(rd) generation partnership project (3GPP) LTE standard. However, the disclosure is not limited to the terms and names and may be similarly applied to systems that comply with other standards. In the disclosure, eNodeB (eNB) may be used interchangeably with gNodeB (gNB) for convenience of explanation. That is, a BS described as an eNB may also indicate a gNB.

The disclosure provides non-interruption handover methods capable of minimizing a data interruption time due to handover or making the data interruption time become 0 ms in a next-generation wireless communication system.

In detail, efficient handover methods provided in the disclosure may have one or more features from among a plurality of features described below.

When a UE receives a handover command message (e.g., a handover command message or an RRC reconfiguration message) from a source BS, the UE performing data transmission or reception (uplink (UL) or downlink (DL) data transmission and reception) to or from the source BS via protocol layers (i.e., a physical (PHY) layer, a medium access control (MAC) layer, a radio link control (RLC) layer, and a PDCP layer) of each of a plurality of first bearers, the UE may configure protocol layers of a plurality of new second bearers corresponding to (e.g., having a same bearer identifier) the protocol layers of the plurality of first bearers, and may perform data transmission or reception (UL or DL data transmission and reception) without discontinuing but maintaining data transmission or reception to or from the source BS via the plurality of first bearers.

After the UE receives a handover command message in the aforementioned situation, the protocol layers (a PHY layer, a MAC layer, an RLC layer, and a PDCP layer) of the plurality of second bearers may be configured for data transmission and reception to and from a target BS, based on bearer configuration information or protocol layer information included in the handover command message.

In the aforementioned situation, the UE may perform data transmission or reception to or from the source BS via the protocol layers of the plurality of first bearers and simultaneously perform a random access procedure on the target BS via a protocol layer (e.g., the MAC layer) of the plurality of second bearers. The random access procedure may include transmission of a preamble, reception of a random access response, transmission of a message 3, reception of a message 4 (e.g., reception of a contention resolution MAC control element (CE) or a UL transmit resource), etc.

The UE may perform data transmission or reception to or from the source BS via the protocol layers of the plurality of first bearers and simultaneously complete the random access procedure on the target BS via the protocol layer (e.g., the MAC layer) of the plurality of second bearers and transmit a handover complete message to the target BS via the protocol layers of the plurality of second bearers.

The UE may perform data transmission or reception to or from the source BS via the protocol layers of the plurality of first bearers and simultaneously complete the random access procedure on the target BS via the protocol layer of the plurality of second bearers, transmit the handover complete message to the target BS via the protocol layers of the plurality of second bearers, and perform data transmission and reception.

When the UE successfully completes the random access procedure with respect to the target BS and then initially receives a UL transmit resource from the target BS, the UE may discontinue data transmission to the source BS via the protocol layers of the plurality of first bearers, switch UL transmission, and then transmit data to the target BS via the plurality of second bearers.

When the UE receives a handover command message, the UE may continuously perform data transmission or reception to or from the source BS via the protocol layers of the plurality of first bearers, and perform a random access procedure on the target BS via the protocol layers of the plurality of second bearers, and when the UE successfully completes the random access procedure and then initially receives a UL transmit resource from the target BS, the UE may discontinue data transmission to the source BS via the protocol layers of the plurality of first bearers, and perform UL data transmission to the target BS only via the protocol layers of the plurality of second bearers. The UE may continuously receive DL data from the source BS via the protocol layers of the plurality of first bearers, and also continuously receive DL data from the target BS via the protocol layers of the plurality of second bearers.

A first bearer and a second bearer may constitute a second PDCP layer architecture, and in the second PDCP layer architecture, the first bearer (e.g., an RLC layer, a MAC layer, or a PHY layer) for the source BS and the second bearer (e.g., an RLC layer, a MAC layer, or a PHY layer) for the target BS may be all connected to one PDCP layer, and UL data may perform transmission via one bearer from among the first bearer or the second bearer of the PDCP layer. That is, before the UE performs a random access procedure on the target BS, successfully completes the random access procedure, and initially receives a UL transmit resource from the target BS, the UE may transmit UL data via the first bearer, and when the UE performs a random access procedure on the target BS, successfully completes the random access procedure, and initially receives a UL transmit resource from the target BS, the UE may discontinue data transmission via the first bearer, may switch the data transmission, and may transmit UL data to the target BS via the second bearer. However, the UE in the second PDCP layer architecture may receive DL data from the source BS or the target BS via the first bearer or the second bearer.

Hereinafter, the disclosure provides efficient handover procedures, without a data interruption time, based on the aforementioned features.

FIG. 1A illustrates an LTE system according to an embodiment.

Referring to FIG. 1A, a RAN of the LTE system includes eNBs (node Bs or base stations) 1 a-05, 1 a-10, 1 a-15, and 1 a-20, a mobility management entity (MME) 1 a-25, and a serving-gateway (S-GW) 1 a-30. A UE 1 a-35 (or a terminal) accesses an external network via the ENB 1 a-05, 1 a-10, 1 a-15, or 1 a-20 and the S-GW 1 a-30.

The ENB 1 a-05, 1 a-10, 1 a-15, or 1 a-20 corresponds to an existing node B of a universal mobile telecommunication system (UMTS). The ENB 1 a-05, 1 a-10, 1 a-15, or 1 a-20 is connected to the UE 1 a-35 through a radio channel and performs complex functions compared to the existing node B. In the LTE system, because all user traffic including a real-time service such as voice over internet protocol (VoIP) is provided via a shared channel, an entity that schedules UE 1 a-35 by gathering state information such as buffer states, available transmit power states, and channel states of the UE 1 a-35 may be necessary, and the ENB 1 a-05, 1 a-10, 1 a-15, or 1 a-20 may operate as the entity.

A single ENB 1 a-05, 1 a-10, 1 a-15, or 1 a-20 generally controls multiple cells. For example, an LTE system may use radio access technology such as orthogonal frequency division multiplexing (OFDM) at a bandwidth of 20 MHz to achieve a data rate of 100 Mbps. The LTE system may also use an adaptive modulation & coding (AMC) scheme to determine a modulation scheme and a channel coding rate in accordance with a channel state of the UE 1 a-35. The S-GW 1 a-30 provides data bearers and generates or removes the data bearers under the control of the MME 1 a-25. The MME 1 a-25 performs a mobility management function and various control functions for the UE 1 a-35 and is connected to the eNBs 1 a-05, 1 a-10, 1 a-15, and 1 a-20.

FIG. 1B illustrates a radio protocol architecture in an LTE system, according to an embodiment.

Referring to FIG. 1B, the radio protocol architecture of the LTE system includes PDCP layers 1 b-05 and 1 b-40, RLC layers 1 b-10 and 1 b-35, MAC layers 1 b-15 and 1 b-30, and PHY layers 1 b-20 and 1 b-25, respectively, for a UE and an eNB. The PDCP layer 1 b-05 or 1 b-40 is in charge of Internet protocol (IP) header compression/decompression, etc. Main functions of the PDCP layer 1 b-05 or 1 b-40 are summarized as shown below.

-   -   Header compression and decompression: robust header compression         (ROHC) only     -   Transfer of user data     -   In-sequence delivery of upper layer packet data units (PDUs) at         PDCP re-establishment procedure for an RLC acknowledge mode (AM)     -   For split bearers in dual connectivity (DC) (only support for an         RLC AM): PDCP PDU routing for transmission and PDCP PDU         reordering for reception     -   Duplicate detection of lower layer service data units (SDUs) at         PDCP re-establishment procedure for an RLC AM     -   Retransmission of PDCP SDUs at handover and, for split bearers         in DC, of PDCP PDUs at a PDCP data-recovery procedure, for an         RLC AM     -   Ciphering and deciphering     -   Timer-based SDU discard in an uplink

The RLC layer 1 b-10 or 1 b-35 performs an automatic repeat request (ARQ) operation by reconfiguring PDCP PDUs to appropriate sizes. Main functions of the RLC layer 1 b-10 or 1 b-35 are summarized as below.

-   -   Transfer of upper layer PDUs     -   Error correction through ARQ (only for AM data transfer)     -   Concatenation, segmentation, and reassembly of RLC SDUs (only         for unacknowledged mode (UM) and AM data transfers)     -   Re-segmentation of RLC data PDUs (only for an AM data transfer)     -   Reordering of RLC data PDUs (only for UM and AM data transfers)     -   Duplicate detection (only for UM and AM data transfers)     -   Protocol error detection (only for an AM data transfer)     -   RLC SDU discard (only for UM and AM data transfers)     -   RLC re-establishment

The MAC layer 1 b-15 or 1 b-30 is connected to multiple RLC layers configured for a single UE and multiplexes RLC PDUs into a MAC PDU and demultiplexes the RLC PDUs from the MAC PDU. Main functions of the MAC layer 1 b-15 or 1 b-30 are summarized as below.

-   -   Mapping between logical channels and transport channels     -   Multiplexing/demultiplexing of MAC SDUs belonging to one or         different logical channels into/from transport blocks (TBs)         delivered to/from the physical layer on transport channels     -   Scheduling information reporting     -   Error correction through hybrid automatic repeat request (HARQ)     -   Priority handling between logical channels of one UE     -   Priority handling between UEs through dynamic scheduling     -   MBMS service identification     -   Transport format selection     -   Padding

The PHY layer 1 b-20 or 1 b-25 channel-codes and modulates upper layer data into OFDM symbols and transmits the OFDM symbols through a radio channel, or demodulates OFDM symbols received through a radio channel and channel-decodes and delivers the OFDM symbols to an upper layer.

FIG. 1C illustrates a next-generation wireless communication system, according to an embodiment.

Referring to FIG. 1C, a RAN of the next-generation wireless communication system (e.g., a new radio (NR) or 5G system) includes a new radio node B (hereinafter, referred to as an NR gNB or an NR base station) 1 c-10 and an NR core network (CN) 1 c-05.

An NR UE or UE 1 c-15 accesses an external network via the NR gNB 1 c-10 and the NR CN 1 c-05.

In FIG. 1C, the NR gNB 1 c-10 corresponds to an eNB of an existing LTE system. The NR gNB 1 c-10 may be connected to the NR UE 1 c-15 through radio channels and may provide superior services compared to an existing node B. In the next-generation wireless communication system, because all user traffic is provided via a shared channel, an entity that schedules UEs (e.g., NR UE 1 c-10) by gathering state information such as buffer states, available transmit power states, and channel states of the UEs (e.g., NR UE 1 c-10) is necessary, and the NR gNB 1 c-10 operates as the entity. A single NR gNB 1 c-10 generally controls multiple cells.

The NR or 5G communication system may have a bandwidth greater than an existing maximum bandwidth to achieve an ultrahigh data rate, compared to a current LTE system, and may use OFDM as radio access technology and may additionally use beamforming technology. The NR or 5G communication system uses an AMC scheme to determine a modulation scheme and a channel coding rate in accordance with a channel state of the NR UE 1 c-15. The NR CN 1 c-05 performs functions such as mobility support, bearer configuration, and quality of service (QoS) configuration. The NR CN 1 c-05 performs a mobility management function and various control functions for the NR UE 1 c-15, and is connected to multiple base stations. The next-generation wireless communication system may cooperate with the existing LTE system, and the NR CN 1 c-05 is connected to an MME 1 c-25 through a network interface. The MME 1 c-25 is connected to an eNB 1 c-30 as an existing base station.

FIG. 1D illustrates a radio protocol architecture of a next-generation wireless communication system, according to an embodiment.

Referring to FIG. 1D, the radio protocol architecture of the next-generation wireless communication system includes NR service data adaptation protocol (SDAP) layers 1 d-01 and 1 d-45, NR PDCP layers 1 d-05 and 1 d-40, NR RLC layers 1 d-10 and 1 d-35, NR MAC layers 1 d-15 and 1 d-30, and NR PHY layers 1 d-20 and 1 d-25, respectively, for a UE and an NR gNB.

Main functions of the NR SDAP layer 1 d-01 or 1 d-45 may include at least some of the following functions.

-   -   Transfer of user plane data     -   Mapping between a QoS flow and a data radio bearer (DRB) for         both DL and UL     -   Marking QoS flow identifiers (IDs) in both DL and UL packets     -   Reflective QoS flow to DRB mapping for the UL SDAP PDUs

With regard to such an SDAP layer, the UE may be configured, via an RRC message, whether to use a header of the SDAP layer or use a function of the SDAP layer for each PDCP layer, each bearer, or each logical channel. In addition, with regard to such an SDAP layer, when an SDAP header is configured, a non-access stratum (NAS) reflective QoS 1-bit indicator and an access stratum (AS) reflective QoS 1-bit indicator of the SDAP header may indicate that the UE is to update or reconfigure mapping information regarding the data bearer and the QoS flow of UL and DL. The SDAP header may include a QoS flow ID indicating a QoS. The QoS information may be used as data processing priority information, scheduling information, etc., for supporting a smooth service.

Main functions of the NR PDCP layer 1 d-05 or 1 d-40 may include at least some of the following functions.

-   -   Header compression and decompression: ROHC only     -   Transfer of user data     -   In-sequence delivery of upper layer PDUs     -   Out-of-sequence delivery of upper layer PDUs     -   PDCP PDU reordering for reception     -   Duplicate detection of lower layer SDUs     -   Retransmission of PDCP SDUs     -   Ciphering and deciphering     -   Timer-based SDU discard in an uplink

The reordering function of the NR PDCP layer 1 d-05 or 1 d-40 refers to a function of sequentially reordering PDCP PDUs received from a lower layer, on a PDCP sequence number (SN) basis. The reordering function of the NR PDCP layer 1 d-05 or 1 d-40 may include delivering the reordered data to an upper layer in order or immediately delivering the reordered data out of order, recording missing PDCP PDUs by reordering the PDCP PDUs, reporting a status of the missing PDCP PDUs to a transmitter, or requesting to retransmit the missing PDCP PDUs.

Main functions of the NR RLC layer 1 d-10 or 1 d-35 may include at least some of the following functions.

-   -   Transfer of upper layer PDUs     -   In-sequence delivery of upper layer PDUs     -   Out-of-sequence delivery of upper layer PDUs     -   Error correction through ARQ     -   Concatenation, segmentation, and reassembly of RLC SDUs     -   Re-segmentation of RLC data PDUs     -   Reordering of RLC data PDUs     -   Duplicate detection     -   Protocol error detection     -   RLC SDU discard     -   RLC re-establishment

The in-sequence delivery of the NR RLC layer 1 d-10 or 1 d-35 refers to sequentially delivering RLC SDUs received from a lower layer to an upper layer. When a single RLC SDU is segmented into multiple RLC SDUs and the multiple RLC SDUs are received, the in-sequence delivery of the NR RLC layer 1 d-10 or 1 d-35 may include reassembling the multiple RLC SDUs and delivering the RLC SDUs, may include reordering received RLC PDUs on an RLC SN or PDCP SN basis, may include recording missing RLC PDUs by reordering the RLC PDUs, reporting a status of the missing RLC PDUs to a transmitter, may include requesting to retransmit the missing RLC PDUs, may include delivering only RLC SDUs previous to a missing RLC SDU, to the upper layer in order, when the missing RLC SDU exists, may include delivering all RLC SDUs received before a timer is started, to the upper layer in order, when a certain timer is expired although a missing RLC SDU exists, or may include delivering all RLC SDUs received up to a current time, to the upper layer in order, when a certain timer is expired although a missing RLC SDU exists. The NR RLC layer 1 d-10 or 1 d-35 may process the RLC PDUs in order of reception regardless of the order of sequence numbers and deliver the RLC PDUs to the NR PDCP layer 1 d-05 or 1 d-40 out-of-sequence delivery, and when the NR RLC layer 1 d-10 or 1 d-35 receives segments, the NR RLC layer 1 d-10 or 1 d-35 may receive the segments received later or stored in a buffer, reconfigure the received segments into a whole RLC PDU, and then process and deliver the whole RLC PDU to the NR PDCP layer 1 d-05 or 1 d-40. The NR RLC layer 1 d-10 or 1 d-35 may not have a concatenation function, and the concatenation function may be performed by the NR MAC layer 1 d-15 or 1 d-30 or be replaced with a multiplexing function of the NR MAC layer 1 d-15 or 1 d-30.

The out-of-sequence delivery of the NR RLC layer 1 d-10 or 1 d-35 includes directly delivering RLC SDUs received from a lower layer, to an upper layer out of order. The out-of-sequence delivery of the NR RLC layer 1 d-10 or 1 d-35 may include, when a single RLC SDU is segmented into multiple RLC SDUs and the multiple RLC SDUs are received, reassembling and delivering the multiple RLC SDUs, or may include storing RLC SNs or PDCP SNs of received RLC PDUs and recording missing RLC PDUs by ordering the RLC PDUs.

The NR MAC layer 1 d-15 or 1 d-30 may be connected to multiple NR RLC layers configured for a single UE, and main functions of the NR MAC layer 1 d-15 or 1 d-30 may include at least some of the following functions.

-   -   Mapping between logical channels and transport channels     -   Multiplexing/demultiplexing of MAC SDUs     -   Scheduling information reporting     -   Error correction through HARQ     -   Priority handling between logical channels of one UE     -   Priority handling between UEs through dynamic scheduling     -   MBMS service identification     -   Transport format selection     -   Padding

The NR PHY layer 1 d-20 or 1 d-25 may channel-code and modulate upper layer data into OFDM symbols and transmit the OFDM symbols through a radio channel, or demodulate OFDM symbols received through a radio channel and channel-decode and deliver the OFDM symbols to an upper layer.

FIG. 1E is a signal flow diagram illustrating a procedure in which a UE switches from an RRC idle mode to an RRC connected mode and configures a connection with a network, according to an embodiment.

Referring to FIG. 1E, when the UE configured to transmit or receive data in the RRC connected mode does not transmit or receive data for a certain reason or for a certain time, a gNB transmits an RRCConnectionRelease message to the UE such that the UE switches to the RRC idle mode in step 1 e-01.

When the UE that is not currently configured for connection (e.g., is an idle-mode UE) has data to be transmitted, the UE performs an RRC connection establishment procedure on the gNB. That is, the UE establishes an inverse direction transmission synchronization with the gNB via a random access process and transmits an RRCSetupRequest message to the gNB in step 1 e-05. The RRCSetupRequest message may include an identifier of the UE and an establishment cause for establishing a connection (e.g., establishmentCause).

In step 1 e-10, the gNB transmits an RRCConnectionSetup message to the UE so that the UE configures an RCC connection.

The RRCConnectionSetup message includes configuration information for each service/bearer/RLC layer or each logical channel or each bearer, and may include information about whether to use ROHC for each bearer/logical channel, ROHC configuration information (e.g., a ROHC version, initial information, etc.), statusReportRequired information (information with which the gNB indicates a PDCP Status report to the UE), and drb-ContinueROHC information (configuration information indicating to continue using ROHC configuration information, which may be transmitted by being included in PDCP layer configuration information (pdcp-config)). The RRCConnectionSetup message may also include RRC connection configuration information. A bearer for an RRC connection may be referred to as a signaling radio bearer (SRB) and is used in transmission and reception of an RRC message that is a control message between the UE and the gNB.

In step 1 e-15, after the UE that has configured RRC connection, the UE transmits an RRCConnectionSetupComplete message to the gNB. The RRCConnectionSetupComplete message may include a control message such as a SERVICE REQUEST message indicating that the UE requests an access mobility management function (AMF) or an MME for bearer configuration for a certain service.

In step 1 e-20, the gNB transmits the SERVICE REQUEST message included in the RRCConnectionSetupComplete message to the AMF or the MME, and the AMF or the MME determines whether to provide the service requested by the UE.

When it is determined that the service requested by the UE is to be provided, the MME or the AMF transmits an INITIAL CONTEXT SETUP REQUEST message to the gNB in step 1 e-25. The INITIAL CONTEXT SETUP REQUEST message includes, for example, QoS information that is to be applied during DRB configuration, and security-related information (e.g., a security key and a security algorithm) that is to be applied to the DRB.

When the gNB does not receive UE capability information from the MME or the AMF, the gNB transmits a UE capability information request message to the UE so as to check the UE capability information in step 1 e-26.

When the UE receives the UE capability information request message, the UE configures, generates, and reports a UE capability information message to the gNB in step 1 e-27. The UE capability information message may include information about which types of handover methods are supported by the UE. For example, the UE may report a UE capability to the gNB via an indicator indicating whether or not the UE supports an efficient handover method (i.e., a dual active protocol stack (DAPS) handover method) described in the disclosure. When the gNB checks the UE capability information and then indicates handover to the UE, the gNB may define an indicator indicating handover in a handover command message, according to each of the handover methods, and may indicate the handover to the UE. For example, the gNB may indicate the efficient handover method (e.g., the DAPS handover method) to the UE, or may configure the DAPS handover method for each bearer (a DRB or an SRB) of the UE. When the gNB configures the DAPS handover method to the UE, the gNB also indicates other handover methods (e.g., a conditional handover method (a method in which, when configurations of a plurality of target cells and a plurality of conditions are configured to the UE and the UE satisfies the conditions in a cell selection procedure or a cell reselection procedure, the UE performs a handover procedure on one target cell) or a handover method without a random access procedure), thereby preventing data loss or a transmission delay which may occur in the handover. The UE may perform a handover procedure on a target gNB according to the handover method indicated in the handover command message.

To configure security with the UE, the gNB exchanges a SecurityModeCommand message in step 1 e-30 and a SecurityModeComplete message in step 1 e-35. When security configuration is complete, the gNB transmits an RRCConnectionReconfiguration message to the UE in step 1 e-40.

The RRCConnectionReconfiguration message includes configuration information for each service/bearer/RLC layer or each logical channel or each bearer, and may include information about whether to use ROHC for each bearer/logical channel, ROHC configuration information (e.g., a ROHC version, initial information, etc.), statusReportRequired information (information with which the gNB indicates a PDCP Status report to the UE), and drb-ContinueROHC information (configuration information indicating to continue and changelessly use ROHC configuration information, which may be transmitted by being included in PDCP layer configuration information (pdcp-config)). The RRCConnectionReconfiguration message may also include RRC connection configuration information. A bearer for RRC connection may be referred to as an SRB and is used in transmission and reception of an RRC message that is a control message between the UE and the gNB.

The RRCConnectionReconfiguration message may include configuration information of a DRB in which user data is to be processed, and the UE configures a DRB by applying the configuration information and transmits an RRCConnectionReconfigurationComplete message to the gNB in step 1 e-45.

When configuration of the DRB with the UE is complete, the gNB transmits an INITIAL CONTEXT SETUP COMPLETE message to the MME or the AMF in step 1 e-50.

In response to the INITIAL CONTEXT SETUP COMPLETE message, the MME or the AMF exchanges an S1 BEARER SETUP message and an S1 BEARER SETUP RESPONSE message with an S-GW to configure an S1 bearer in step 1 e-55 and 1 e-60. The S1 bearer is a connection for data transmission, which is configured between the S-GW and the gNB, and corresponds to the DRB in a one-to-one manner.

When all of these processes are completed, the UE transmits or receives data to or from the gNB via the S-GW in step 1 e-65 and 1 e-70. As such, a general data transmission process roughly includes three operations of RRC connection configuration, security configuration, and DRB configuration.

The gNB may transmit an RRCConnectionReconfiguration message to the UE in order to renew, add, or change configuration for a certain reason in step 1 e-75.

Herein, a bearer may include an SRB and a DRB. The SRB is mainly used to transmit and receive an RRC message of an RRC layer, and the DRB is mainly used to transmit and receive a plurality of items of user layer data (or user plane data). A UM DRB is a DRB that uses an RLC layer operating in a UM, and an AM DRB is a DRB that uses an RLC layer operating in an AM.

FIG. 1F is a signal flow diagram illustrating signaling procedures for performing a handover in a next-generation wireless communication system, according to an embodiment.

Referring to FIG. 1F, a UE 1 f-01 in an RRC connected mode state reports a cell measurement report to a current source gNB 1 f-02 in a periodic manner or when a particular event is satisfied in step 1 f-05. The source gNB 1 f-02 determines, based on the cell measurement report, whether the UE 1 f-01 is to perform a handover to an adjacent cell. The handover refers to a technology of switching a source BS to another BS (or another cell in a same BS), the source BS providing a service to a UE in a connected mode state.

When the source gNB 1 f-02 determines the handover, the source gNB 1 f-02 requests the handover by transmitting a handover request message (e.g., a handover preparation information message) to a new BS to provide a service to the UE 1 f-01, i.e., a target gNB 1 f-03, in step 1 f-10.

When the target gNB 1 f-03 accepts the handover request, the target gNB 1 f-03 transmits a handover request acknowledgement (Ack) message (e.g., a handover command message) to the source gNB 1 f-02 in step 1 f-15.

In response to the handover request Ack message, the source gNB 1 f-02 transmits, to the UE 1 f-01, the handover command message (e.g., an RRCReconfiguration message included in a Dedicated Control Channel (DCCH) of the handover request Ack message) in step 1 f-20.

The source gNB 1 f-02 extracts the handover command message from the handover request Ack message received from the target gNB 1 f-03 and transmits the handover command message to the UE 1 f-01 by using an RRC Connection Reconfiguration message in step 1 f-20.

In accordance with an aspect of the disclosure, a method is provided for determining an efficient DAPS handover method by using the handover preparation information message in step 1 f-10 and the handover command message in step 1 f-15, when the source gNB 1 f-02 transmits the handover preparation information message in step 1 f-10 and, in response thereto, the target gNB 1 f-03 transmits the handover command message to the source gNB 1 f-02 in step 1 f-15.

In accordance with an embodiment, hereinafter Embodiment 1, an entity for determining a DAPS handover method may be a source BS. When the source BS requests a target BS for the DAPS handover method, the target BS may always indicate or perform the DAPS handover method.

The source BS may indicate, to the target BS, by defining a new indicator in the handover preparation information message, that the source BS is to perform the DAPS handover method, and may request the DAPS handover method. The handover preparation information message may include current bearer configuration information of a UE, security key information, cell group configuration information, UE capability information, etc. The source BS is configured to pre-share a capability of the target BS and thus may know in advance whether the target BS supports the DAPS handover method. The source BS may indicate, to the target BS, that the source BS is to perform the DAPS handover method, may indicate, to the target BS, that the source BS may perform data forwarding fast or early, and may indicate to the target BS to prepare to receive data forwarding and fast process the data forwarding. The source BS may request the DAPS handover method for each bearer (e.g., a DRB or an SRB).

When the target BS receives the handover preparation information message and identifies that an indicator requesting the DAPS handover method is included therein, and the target BS configures an RRCReconfiguration message to indicate a handover to the UE, the target BS may add, to the RRCReconfiguration message, an indicator requesting the DAPS handover method, bearer configuration information required for the UE to perform the DAPS handover method, bearer configuration information, security key information, cell group configuration information, or system information. The target BS may add the RRCReconfiguration message to a DL-DCCH message of a handover command message and may transmit the handover command message to the source BS. The target BS may perform indication of the DAPS handover method for each bearer.

When the source BS receives the handover command message, the source BS may extract the RRCReconfiguration message included in the handover command message or may transmit the RRCReconfiguration message to the UE, and thus may indicate a handover. The source BS may identify the indicated DAPS handover method for each bearer, and may perform the DAPS handover method for each bearer.

In accordance with another embodiment, hereinafter Embodiment 2, an entity for determining a DAPS handover method may be a target BS. When a source BS requests the target BS for the DAPS handover method through an indicator, the target BS may reject or accept the request or may indicate another handover method to the source BS via a handover command message indicating the other handover method.

The source BS may indicate, to the target BS, by defining a new indicator in the handover preparation information message, that the source BS is to perform the DAPS handover method, and may request the DAPS handover method. The handover preparation information message may include current bearer configuration information of a UE, security key information, cell group configuration information, UE capability information, etc. The source BS is configured to pre-share a capability of the target BS and thus may know in advance whether the target BS supports the DAPS handover method. The source BS may indicate, to the target BS, that the source BS is to perform the DAPS handover method, may indicate, to the target BS, that the source BS may fast perform early data forwarding, and may indicate the target BS to prepare to receive data forwarding and fast process the data forwarding. The source BS may make a request for the DAPS handover method for each bearer.

When the target BS receives the handover preparation information message and identifies that an indicator requesting the DAPS handover method is included therein, the target BS may reject or accept the request for the DAPS handover method or may indicate another handover method to the source BS, based on whether the target BS can support the DAPS handover method, an amount of current transmit resources, or scheduling. The target BS may add, to a handover command message, an indicator to reject the request for the DAPS handover method, an indicator to accept the request for the DAPS handover method, or an indicator to indicate the other handover method, and may transmit the handover command message. When the target BS accepts the DAPS handover request and configures an RRCReconfiguration message to indicate handover to the UE, the target BS may configure the RRCReconfiguration message by including the indicator indicating the DAPS handover method in the RRCReconfiguration message. When the target BS rejects the DAPS handover request, the target BS may configure the RRCReconfiguration message by including an indicator indicating another handover method in the RRCReconfiguration message and including, in the RRCReconfiguration message, bearer configuration information for the UE to perform the DAPS handover method or the other handover method, bearer configuration information, security key information, cell group configuration information, or system information. The target BS may add the RRCReconfiguration message to a DL-DCCH message of a handover command message and may transmit the handover command message to the source BS. The target BS may perform indication of the DAPS handover method for each bearer.

When the source BS receives the handover command message, the source BS may check an indicator included in the handover command message and may identify whether the request for the DAPS handover method is accepted or rejected. When the request for the DAPS handover method is accepted, the source BS may also perform the DAPS handover method, may extract the RRCReconfiguration message included in the handover command message, may transmit the RRCReconfiguration message to the UE, and may indicate the handover. When the source BS checks the indicator included in the handover command message and the request for the DAPS handover method is rejected or the other handover message is indicated, the source BS may perform the other handover method indicated by the target BS. The source BS may extract the RRCReconfiguration message included in the handover command message or may transmit the RRCReconfiguration message to the UE, and may indicate the handover.

Alternatively, even when a separate indicator is not present in the handover command message, the source BS may check a type of a handover message indicated by the target BS by reading the RRCReconfiguration message included in the handover command message, and may identify whether the request for the DAPS handover method is accepted or rejected. The source BS may also perform a handover method (e.g., the DAPS handover method or the other handover method) indicated in the RRCReconfiguration message. The source BS may identify the indicated DAPS handover method for each bearer, and may perform the DAPS handover method for each bearer.

In accordance with another embodiment, hereinafter Embodiment 3, an entity for determining a DAPS handover method is a target BS. The target BS may check the capability of a UE, and may determine a handover method (e.g., a DAPS handover method) according to whether the target BS can support the DAPS handover method, an amount of current transmit resources, or scheduling.

A source BS may add, to the handover preparation information message, current bearer configuration information of a UE, security key information, cell group configuration information, UE capability information, etc., and may transmit the handover preparation information message to request the target BS for handover. The source BS is configured to pre-share a capability of the target BS, and thus, may know in advance whether the target BS supports the DAPS handover method. When the target BS indicates to perform the DAPS handover method, the source BS may perform data forwarding fast or early.

The target BS may receive the handover preparation information message, and may determine the handover method (e.g., the DAPS handover method) according to UE capability information, whether the target BS can support the DAPS handover method, an amount of current transmit resources, and/or scheduling. When the target BS determines the DAPS handover method for the handover command message, the target BS may add, to the handover command message, an indicator indicating the DAPS handover method, and may transmit the handover command message. When the target BS determines the DAPS handover and configures an RRCReconfiguration message to indicate handover to the UE, the target BS may configure the RRCReconfiguration message by including the indicator indicating the DAPS handover method in the RRCReconfiguration message. When the target BS determines another handover method, which is different from the DAPS handover, the target BS may configure the RRCReconfiguration message by including an indicator indicating the other handover method in the RRCReconfiguration message and including, in the RRCReconfiguration message, bearer configuration information necessary for the UE to perform the DAPS handover method or the other handover method, bearer configuration information, security key information, cell group configuration information, or system information. The target BS may also add the RRCReconfiguration message to a DL-DCCH message of a handover command message and may transmit the handover command message to the source BS. The target BS may perform indication of the DAPS handover method for each bearer.

When the source BS receives the handover command message, the source BS may check an indicator included in the handover command message and thus may identify whether the DAPS handover method is determined. When the DAPS handover method is indicated, the source BS may perform the DAPS handover method, may extract the RRCReconfiguration message included in the handover command message, may transmit the RRCReconfiguration message to the UE, and may indicate the handover. When the source BS checks the indicator included in the handover command message and the DAPS handover method is not determined or the other handover message is indicated, the source BS may perform the other handover method indicated by the target BS. The source BS may extract the RRCReconfiguration message included in the handover command message or may transmit the RRCReconfiguration message to the UE, and thus may indicate the handover. Alternatively, even when a separate indicator is not present in the handover command message, the source BS may check a type of a handover message indicated by the target BS by reading the RRCReconfiguration message included in the handover command message, and may identify whether the DAPS handover method is determined. When the other handover method is indicated, the source BS may perform the indicated other handover method. The source BS may identify the indicated DAPS handover method for each bearer, and may perform the DAPS handover method for each bearer.

Different embodiments may be derived by combining methods of Embodiment 1, Embodiment 2, or Embodiment 3.

A BS may indicate, via the RRCReconfiguration message, an efficient handover method (e.g., a DAPS handover method) to the UE, or in another method, the BS may configure the DAPS handover method for each bearer of the UE. For example, a new indicator indicating the efficient handover method in bearer configuration information, PDCP configuration information, or RLC configuration information for each bearer identifier or each logical channel identifier may be defined in the RRCReconfiguration message, and the BS may indicate, by using the new identifier, the efficient handover method for each bearer or logical channel identifier to the UE. When the BS configures the DAPS handover method to the UE, the BS may also indicate other handover methods (e.g., a conditional handover method in which, when configurations of a plurality of target cells and a plurality of conditions are configured to the UE and the UE satisfies the conditions in a cell selection procedure or a cell reselection procedure, the UE performs a handover procedure on one target cell, or a handover method without a random access procedure), thereby preventing data loss or a transmission delay which may occur in handover.

In response to the RRCReconfiguration message in step 1 f-20, the UE 1 f-01 discontinues or continues data transmission and reception to and from the source gNB 1 f-02 and starts a T304 timer in step 1 f-25. When the UE 1 f-01 cannot succeed in handover to the target gNB 1 f-03 for a preset time (e.g., when T304 timer is expired), the UE 1 f-01 returns to original configuration of the UE 1 f-01, and the UE 1 f-01 transitions to an RRC idle state. The UE 1 f-01 may trigger an RRC connection re-establishment procedure, and when an efficient handover method is configured according to another method and connection to the source gNB 1 f-02 is active, the UE 1 f-01 may fall back and report its handover failure to the source gNB 1 f-02 and may re-establish connection.

The source gNB 1 f-02 provides an SN status of UL/DL data for each bearer (e.g., for each RLC UM bearer or each RLC AM bearer), and, when DL data or UL data is present, the source gNB 1 f-02 transmits the DL data or the UL data to the target gNB 1 f-03 in steps 1 f-30 and 1 f-35. The UE 1 f-01 attempts a random access to a target cell (e.g., target gNB 1 f-03) indicated by the source gNB 1 f-02 in step 1 f-40. The UE 1 f-01 performs the random access to notify switching of the UE 1 f-01 to the target cell and simultaneously to match UL synchronization, via the handover. For the random access, the UE 1 f-01 transmits, to the target cell, a preamble that corresponds to a preamble ID provided by the source gNB 1 f-02 or corresponds to a randomly-selected preamble. After a certain number of subframes after the preamble is transmitted, the UE 1 f-01 monitors whether a random access response (RAR) message is transmitted from the target cell. A time interval for monitoring the RAR message may be referred to as a RAR window.

When the RAR message is received during the RAR window in step 1 f-45, the UE 1 f-01 transmits a handover complete message in an RRC Reconfiguration Complete message to the target gNB 1 f-03 in step 1 f-55.

When the UE 1 f-01 successfully receives the RAR message from the target gNB 1 f-03, the UE 1 f-01 ends T304 timer in step 1 f-50. To switch a path of bearers that is configured for the source gNB 1 f-02, the target gNB 1 f-03 requests a core network 1 f-04 (e.g., MME/S-GW/AMF) for a path switch of the bearers in steps 1 f-60 and 1 f-65, and indicates to the source gNB 1 f-02 to discard UE context of the UE 1 f-01 in step 1 f-70.

In step 1 f-71, the target gNB 1 f-03 transmits an RRC message (e.g., an RRCReconfiguration message) to the UE 1 f-01 and may indicate to the UE 1 f-01, by using an indicator, to release the connection with the source gNB 1 f-02. Alternatively, the target gNB 1 f-03 may transmit MAC control information, RLC control information, or PDCP control information to the UE 1 f-01 and indicate to the UE 1 f-01 to release the connection with the source gNB 1 f-02. Accordingly, the UE 1 f-01 attempts, at a start point of the RAR window, to receive data from the target gNB 1 f-03, and after the RAR message is received, the UE 1 f-01 transmits an RRC Reconfiguration Complete message and receives a DL transmit resource or a UL transmit resource, thereby starting data transmission and reception to and from the target gNB 1 f-03.

In accordance with an aspect of the disclosure, non-interruption handover methods are provided for minimizing a data interruption time due to handover or making the data interruption time become 0 ms in the next-generation wireless communication system.

A UE may configure a plurality of first bearers with a source BS and may perform data transmission and reception via protocol layers (a PHY layer, a MAC layer, an RLC layer, a PDCP layer, etc.) of each of the plurality of first bearers. However, for convenience of description, it is assumed, in drawings and descriptions, that the UE has one bearer.

FIG. 1G illustrates an efficient handover method for minimizing a data interruption time due to a handover, according to an embodiment.

Referring to FIG. 1G, according to Embodiment 1, a UE 1 g-20 transmits or receives data to or from a source BS 1 g-05 in a first operation 1 g-01 and then receives a handover command message from the source BS 1 g-05. When receiving the handover command message according to a handover method indicated by the handover command message (e.g., an RRCReconfiguration message), the UE 1 g-20 may release connection with the source BS 1 g-05, may perform a random access procedure on a target BS 1 g-10, and may perform a handover procedure. To minimize a data interruption time occurring during the handover based on the indicated handover method, the UE 1 g-20 may continuously transmit and receive data to and from the source BS 1 g-05.

In a second operation 1 g-02, when the UE 1 g-20 performs the random access procedure on the target BS 1 g-10 by using the handover method indicated by the handover command message received from the source BS 1 g-05, transmits a preamble to the target BS 1 g-10, or initially transmits data in a UL transmit resource by using a physical uplink control channel (PUCCH) or a physical uplink shared channel (PUSCH) transmit resource, the UE 1 g-20 may discontinue data transmission and reception to and from the source BS 1 g-05.

In a third operation 1 g-03, the UE 1 g-20 may complete the random access procedure with respect to the target BS 1 g-10, may transmit a handover complete message to the target BS 1 g-10, and may start data transmission and reception to and from the target BS 1 g-10.

FIG. 1H illustrates an efficient handover method for minimizing a data interruption time due to a handover, according to an embodiment.

Referring to FIG. 1H, according to Embodiment 2, a UE 1 h-20 may transmit or receive data to or from a source BS 1 h-05 in first operation 1 h-01 and then receive a handover command message from the source BS 1 h-05. When the source BS 1 h-05 indicates, in the handover command message, the efficient handover method according to Embodiment 2 (e.g., a DAPS handover method) or indicates the efficient handover method for each bearer, even when the UE 1 h-20 has received the handover command message, the UE 1 h-20 may continuously transmit and receive data to and from the source BS 1 h-05 via protocol layers 1 h-22 of a first bearer so as to minimize a data interruption time occurring during handover.

When the RRC layer of the UE 1 h-20 identifies, in the handover command message, an indication with respect to the efficient handover method according to Embodiment 2 or identifies an identifier with respect to the DAPS handover method for each bearer, the RRC layer may provide the indicator to a PDCP layer corresponding to each bearer or a bearer for which the DAPS handover method is indicated. In response to the indicator, the PDCP layer may switch a first PDCP layer architecture (e.g., the first PDCP layer architecture 1 i-11 or 1 i-12 in FIG. 1I) to a second PDCP layer architecture (e.g., the second PDCP layer architecture 1 i-20 in FIG. 1I).

First operation 1 h-01 of FIG. 1H may be described as an operation in which the UE 1 h-20 receives a handover command message (RRCReconfiguration message) from a BS. When the PDCP layer transits to the second PDCP layer architecture 1 i-20 according to configuration included in the received handover command message, protocol layers (a PHY layer, a MAC layer, an RLC layer, or a PDCP layer) 1 h-21 of a second bearer for a target BS 1 h-10 may be pre-configured or pre-established, a security key for the target BS 1 h-10 may be derived and updated, and header (or data) compression context for the target BS 1 h-10 may be configured. The UE 1 h-20 may receive the handover command message from the source BS 1 h-05.

When the handover command message indicates the DAPS handover method, when the handover command message indicates a DAPS handover method for particular bearers, or when a PDCP realignment timer value is newly configured, the UE 1 h-20 may switch a PDCP layer from the first PDCP layer architecture or function to the second PDCP layer architecture or function with respect to each bearer or a bearer for which the DAPS handover method is indicated. In this case, the UE 1 h-20 may update a variable for realignment to a PDCP SN or COUNT value, which is predicted to be received next time, may stop a realignment timer, and may restart the realignment timer.

When the UE 1 h-20 receives the handover command message (e.g., the RRCReconfiguration message), the RRC layer of the UE 1 h-20 may start a first timer (e.g., T304). The first timer may be stopped when the UE 1 h-20 performs a random access procedure on the target BS 1 h-10 so as to perform handover and the random access procedure is successfully completed (e.g., when a first condition proposed in the disclosure is satisfied). When the handover fails and thus the first timer expires, when connection to the source BS 1 h-05 is active, the UE 1 h-20 may fall back and report its handover failure to the source BS 1 h-05 and may attempt connection recovery, but when connection to the source BS 1 h-05 is inactive, the UE 1 h-20 may perform an RRC connection re-establishment procedure.

The handover command message may be configured and established such that a second bearer has the same identifier as a first bearer so that a data interruption time does not occur in each bearer. In Embodiment 2, a PDCP layer of a first bearer and a PDCP layer of a second bearer may logically operate as one PDCP layer, and detailed descriptions about the operation will be provided below with reference to FIG. 1I.

In Embodiment 2, when the UE 1 h-20 is configured to transmit UL data to both the source BS 1 h-05 and the target BS 1 h-10, to avoid a coverage lessening problem due to insufficient transmission power of the UE 1 h-20 or to prevent link selection by which, when the UE 1 h-20 transmits UL data, the UE 1 h-20 has to determine to which BS the UE 1 h-20 has to request a transmit resource and to transmit the UL data, the transmission of the UL data may be performed by only one of the source BS 1 h-05 and the target BS 1 h-10. For example, when the UE 1 h-20 does not have a capability (dual uplink transmission) of simultaneously transmitting UL data to different BSs at different frequencies or a same frequency, the UE 1 h-20 may transmit the UL data to only one of the source BS 1 h-05 and the target BS 1 h-10 within one time unit. Therefore, the UE 1 h-20 may perform a scheduling request to only one of the source BS 1 h-05 or the target BS 1 h-10, may transmit a report (e.g., a buffer status report) about a size of a plurality of items of data to be transmitted by the PDCP layer to only one of the source BS 1 h-05 or the target BS 1 h-10, may receive a UL transmit resource, and may transmit UL data to only one BS. Even when the UE 1 h-20 receives a handover command message from the source BS 1 h-05, the UE 1 h-20 may not initialize a MAC layer of a first bearer so as to prevent data loss by continuing data transmission and reception by retransmitting HARQ. An RLC layer in an AM mode may continuously perform RLC retransmission. Alternatively, when the handover command message indicates, for each bearer, Embodiment 2 (the DAPS handover method), the UE 1 h-20 may continuously transmit or receive data to or from the source BS 1 h-05 with respect to only a PDCP layer, an RLC layer, or a MAC layer, which corresponds to a bearer or a logical channel identifier for which Embodiment 2 is indicated in the handover command message, or with respect to only data corresponding to the bearer or the logical channel identifier. When the first condition proposed in the disclosure is satisfied (e.g., when UL data transmission is switched to the target BS 1 h-10), the UE 1 h-20 may continuously transmit or receive RLC control data (an RLC status report), PDCP control data (an ROHC feedback or a PDCP status report), or HARQ retransmission to or from the source BS 1 h-05 with respect to only the PDCP layer, the RLC layer, or the MAC layer, which corresponds to the bearer or the logical channel identifier for which Embodiment 2 is indicated in the handover command message. When the handover command message indicates, for each bearer, Embodiment 2, the UE 1 h-20 may discontinue transmission or reception of data to or from the source BS 1 h-05 with respect to a PDCP layer, an RLC layer, or a MAC layer, which corresponds to a bearer or a logical channel identifier for which Embodiment 2 is not indicated in the handover command message.

In Embodiment 2, in second operation 1 h-02, even when performing the random access procedure on the target BS 1 h-10 indicated by the handover command message via the protocol layers of the second bearer, the UE 1 h-20 may continue data transmission or reception to or from the source BS 1 h-05 via the protocol layers 1 h-22 of the first bearer. Second operation 1 h-02 may be described as an operation in which the UE 1 h-20 performs a cell selection procedure or a cell reselection procedure, and performs a random access procedure on a target cell indicated by the handover command message (e.g., an RRCReconfiguration message) received from the source BS 1 h-05.

When the first condition to be described below is satisfied in third operation 1 h-03, the UE 1 h-20 may discontinue UL data transmission to the source BS 1 h-05 via the protocol layers 1 h-22 of the first bearer and may transmit the UL data to the target BS 1 h-10 via the protocol layers 1 h-21 of the second bearer. The UE 1 h-20 may continuously receive DL data from the source BS 1 h-05 and the target BS 1 h-10 via the protocol layers 1 h-22 of the first bearer and the protocol layers 1 h-21 of the second bearers. In the third operation 1 h-03, the UE 1 h-20 satisfies the first condition and thus switches UL transmission from the source BS 1 h-05 to the target BS 1 h-10. For example, the UE 1 h-20 may transmit UL data to the source BS 1 h-05 via the first bearer until the UE 1 h-20 satisfies the first condition, and when the UE 1 h-20 satisfies the first condition, the UE 1 h-20 may discontinue transmission of the UL data to the source BS 1 h-05 via the first bearer, and start transmission of the UL data to the target BS 1 h-10 via the second bearer. In the second PDCP layer architecture, when the PDCP layer transmits UL data via the first bearer and then satisfies the first condition and thus receives an indicator from a lower layer (when the MAC layer succeeds in a random access procedure on a target BS) or an upper layer (when a first timer is expired in the RRC layer, the PDCP layer may discontinue transmission of the UL data via the first bearer, may perform switching, and may start transmission of the UL data via the second bearer). As in the PDCP layer structure of FIG. 1I, a reception PDCP layer 1 h-21 of the second bearer and a reception PDCP layer 1 h-22 of the first bearer may operate as one entity, and the PDCP layer structure proposed with reference to FIG. 1I may continuously receive data from the source BS 1 h-05 or the target BS 1 h-10 without interruption by using stored, transceived data, SN information, or information such as header compression and decompression context.

The first condition may be one of the conditions described below. The first conditions below propose a UL data transmission switching time point at which a transmit resource may be most efficiently used, and a data interruption time may be minimized as much as possible.

The first condition may be satisfied when the UE successfully completes a random access procedure on the target BS via the layers (e.g., a MAC layer) of the second bearer, when the UE successfully completes the random access procedure on the target BS via the layers (e.g., the MAC layer) of the second bearer and receives allocation of a first UL transmit resource from the target BS, or when a UL transmit resource is first indicated to the UE.

For example, when the UE receives a handover command message from the source BS and receives an indication of a random access to the target BS, and the indicated random access is a contention free random access (CFRA) (e.g., when a predefined preamble or a UE-cell identifier (e.g., a cell-radio network temporary identifier (C-RNTI) is allocated), it may be determined that the random access procedure is successfully completed when the UE transmits the predefined preamble to a cell of the target BS and receives a RAR message. Therefore, when the UE receives a first UL transmit resource allocated, or included, or indicated in the RAR message, it may be determined that the first condition is satisfied.

As another method, when the UE first receives a UL transmit resource after the UE receives the RAR message, it may be determined that the first condition is satisfied.

When the UE receives a handover command message from the source BS and receives an indication of a random access to the target BS, when the indicated random access is a contention-based random access (CBRA) (e.g., when a predefined preamble or a UE-cell identifier (e.g., C-RNTI) is not allocated), it may be determined that the random access procedure on the target BS is successfully completed when the UE transmits a preamble (e.g., a random preamble) to a cell of the target BS and receives a RAR message, transmits a message3 (e.g., a handover complete message) to the target BS by using a UL transmit resource allocated, or included, or indicated in the RAR message, and receives, from the target BS and via a message4, a contention resolution MAC CE indicating resolution of contention or receives a UL transmit resource via a physical downlink control channel (PDCCH) corresponding to a C-RNTI of the UE. Therefore, when the UE monitors the PDCCH and first receives or is first indicated with the UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that the first condition is satisfied.

As another method, when a size of the UL transmit resource allocated in the RAR message is sufficient and thus the UE can transmit the message3 and UL data, the UE may determine that the UE first receives a UL transmit resource and thus may determine that the first condition is satisfied. In other words, when the UE receives a RAR message, the UE may determine that the UE first receives the UL transmit resource and may determine that the first condition is satisfied.

When a handover method (e.g., a RACH-less handover) that does not require a random access procedure is also indicated in the handover command message received by the UE, and when the handover command message includes a UL transmit resource with respect to the target BS, the UE transmits a message3 (e.g., a handover complete message or an RRCReconfigurationComplete message) by using the UL transmit resource with respect to the target BS. When the UE receives, from the target BS, a UE Identity Confirmation MAC CE via a message4, or receives a UL transmit resource via a PDCCH corresponding to a C-RNTI of the UE, it may be determined that a random access procedure is successfully completed and the first condition is satisfied.

As another method, when the random access procedure is successfully completed and the UE performs PDCCH monitoring and receives a first UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that the first condition is satisfied.

When the handover command message does not include the UL transmit resources with respect to the target BS, the UE performs PDCCH monitoring on the target BS (or a cell). When the UE receives a UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, or transmits a message3 (e.g., a handover complete message or an RRCReconfigurationComplete message) by using the UL transmit resource, and receives a UE Identity Confirmation MAC CE from the target BS or receives a UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that a random access procedure is successfully completed and the first condition is satisfied.

As another method, when the random access procedure is successfully completed and the UE performs PDCCH monitoring and receives a first UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that the first condition is satisfied.

Hereinafter, methods are described for efficiently switching UL data from a source BS to a target BS, the methods being performed when the DAPS handover method proposed in the disclosure is performed. A MAC layer or an RRC layer of a second bearer for the target BS may check or identify whether the first condition corresponding to the second bearer is satisfied, by using one or a combination of the methods below.

-   -   First method: When an RRCReconfiguration message received by the         UE indicates DAPS handover, the UE may configure the MAC layer         for the target BS corresponding to the second bearer, and the         MAC layer may perform a random access procedure and identify         whether the first condition is satisfied. When the first         condition is satisfied, the MAC layer may indicate, by using an         indicator, an upper layer (e.g., a PDCP layer) to switch UL data         transmission from the source BS via a first bearer to the target         BS via the second bearer in the DAPS handover method.     -   Second method: When an RRCReconfiguration message received by         the UE indicates DAPS handover, the UE may configure the MAC         layer for the target BS corresponding to the second bearer, and         the MAC layer may perform a random access procedure and identify         whether the first condition is satisfied. When the first         condition is satisfied, the MAC layer may indicate, to an upper         layer (e.g., an RRC layer), that the first condition is         satisfied. The upper layer may indicate, by using an indicator,         a lower layer (e.g., a PDCP layer) to switch UL data         transmission from the source BS via a first bearer to the target         BS via the second bearer in the DAPS handover method. When the         first condition proposed in the disclosure is satisfied or a         random access procedure on the target BS is successfully         performed, the upper layer may stop a first timer, and when the         first timer is stopped, the RRC layer may indicate, by using an         indicator, the PDCP layer to switch the UL data transmission.     -   Third method: When an RRCReconfiguration message received by the         UE indicates a DAPS handover, the UE may configure the MAC layer         for the target BS corresponding to the second bearer, and when         the RRC layer of the UE indicates, by using an indicator, a         lower layer to perform the DAPS handover, the MAC layer may         perform a random access procedure and may check whether the         first condition is satisfied. When the first condition is         satisfied, the MAC layer may indicate, by using an indicator, an         upper layer to switch UL data transmission from the source BS         via a first bearer to the target BS via the second bearer in the         DAPS handover method.     -   Fourth method: When an RRCReconfiguration message received by         the UE indicates the DAPS handover, the UE may configure the MAC         layer for the target BS corresponding to the second bearer, and         when the RRC layer of the UE indicates, by using an indicator, a         lower layer (e.g., the MAC layer) to perform the DAPS handover,         the MAC layer may perform a random access procedure and may         check whether the first condition is satisfied. When the first         condition is satisfied, the MAC layer may indicate, to an upper         layer (e.g., an RRC layer), that the first condition is         satisfied. In a case where the indicator is identified, the         upper layer (e.g., the RRC layer) may stop the first timer when         the first condition proposed in the disclosure is satisfied or         the random access procedure on the target BS is successfully         performed. The upper layer (e.g., the RRC layer) may indicate,         by using an indicator, a lower layer (e.g., a PDCP layer) to         switch UL data transmission from the source BS via a first         bearer to the target BS via the second bearer in the DAPS         handover method.

When the PDCP layer receives an indicator indicating that the first condition is satisfied or an indicator indicating switching UL data transmission from the source BS to the target BS, from the upper layer (e.g., the RRC layer) or the lower layer (e.g., the MAC layer) according to the first method, the second method, the third method, or the fourth method (e.g., when the DAPS handover method is indicated), the PDCP layer may perform a protocol layer operation as described below in order to efficiently perform switching of UL data transmission, and may perform one or more operations from among operations below in order to prevent data loss due to the UL data transmission. The operations below may be applied to the PDCP layer connected to an AM DRB or a UM DRB (e.g., an RLC layer operating in an AM mode or an RLC layer operating in a UM mode).

Before the first condition is satisfied or before the indicator indicating that the first condition is satisfied is received, the PDCP layer may indicate, to the MAC layer of the first bearer for the source BS, that there is data to be transmitted by indicating a size or amount (e.g., a PDCP data volume) of the data to be transmitted when a buffer stores the data to be transmitted, and may perform UL data transmission to the source BS. The MAC layer of the first bearer for the source BS may perform a scheduling request or a buffer status report procedure to receive allocation of a UL transmit resource from the source BS. However, when the first condition is satisfied or the indication that the first condition is satisfied is received, UL data transmission may be switched to the target BS in a manner described below.

To switch UL data transmission from the first bearer for the source BS to the second bearer for the target BS, the PDCP layer may indicate, to the MAC layer of the first bearer for the source BS, that a size or amount of data to be transmitted is 0 (or none). That is, the PDCP layer may indicate, to the MAC layer of the first bearer, that a data volume (a PDCP data volume) of the PDCP layer is 0, thereby indicating that there is no more data to be transmitted (even when the buffer actually stores a plurality of items of data to be transmitted, in order to switch UL data transmission, the PDCP layer may indicate, to the MAC layer of the first bearer for the source BS, that there is no more data to be transmitted). However, when the handover method of Embodiment 2 is indicated or when a bearer for which the handover method of Embodiment 2 is indicated, and RLC control data (e.g., an RLC status report) or PDCP control data (e.g., a PDCP status report or an ROHC feedback) is generated, the PDCP layer may indicate a data volume corresponding to the RLC control data or the PDCP control data to the MAC layer and may perform data transmission to the source BS.

The PDCP layer connected to an AM DRB (e.g., an RLC layer operating in an AM mode) (all pre-stored PDCP PDUs are discarded (e.g., PDCP SDUs are not discarded to prevent loss of original data)) may perform, based on header context for the target BS, a new header compression procedure on a plurality of items of data (e.g., PDCP SDUs of the buffer) in ascending order of COUNT values (or PDCP SNs) allocated before the first condition is satisfied or the indication that the first condition is satisfied is received, the ascending order starting from first data (e.g., a PDCP SDU) for which successful delivery is not acknowledged by lower layers (e.g., the RLC layer corresponding to the first bearer for the source BS). The PDCP layer may re-perform, by applying security keys for the target BS, an integrity procedure or a ciphering procedure on the plurality of items of data on which the new header compression procedure has been performed, may configure a PDCP header, and may provide the PDCP header with the data (e.g., resulting PDCP PDU data) to a lower layer (e.g., an RLC layer of the second bearer for the target BS), thereby performing retransmission or transmission. That is, the PDCP layer performs accumulated retransmission on data starting from first data for which successful delivery is not acknowledged.

As another method, when the PDCP layer performs retransmission, the PDCP layer may perform retransmission only on a plurality of items of data for which successful delivery is not acknowledged by lower layers (e.g., the RLC layers of the first bearer for the source BS). The PDCP layer connected to the AM DRB (e.g., an RLC layer operating in the AM mode) (e.g., PDCP PDUs that are stored to be transmitted to the source BS via a first protocol layer previously connected to the PDCP layer are all discarded (e.g., PDCP SDUs may not be discarded to prevent loss of original data)) may perform, by applying header compression (or data compression) protocol context or security key corresponding to the target BS, a new header or data compression procedure on only a plurality of items of data (e.g., the PDCP SDUs) for which successful delivery is not acknowledged by lower layers (e.g., the RLC layers) that are the first protocol layer for the source BS, based on COUNT values (or PDCP SNs) allocated before the first condition is satisfied or the indicator indicating that the first condition is satisfied is received. The PDCP layer may re-perform an integrity procedure or a ciphering procedure on the plurality of items of data on which the new header or data compression procedure has been performed, may configure a PDCP header, and may provide the PDCP header with the data to a lower layer that is a second protocol layer for transmission to the target BS, thereby performing retransmission or transmission. That is, to prevent a waste of transmit resources, the PDCP layer may perform selective retransmission on the plurality of items of data for which successful delivery is not acknowledged.

As another method, the transmission or the retransmission may be performed after lower layers (e.g., a transmission or reception RLC layer or MAC layer) that are the first protocol layer for transmitting data to the source BS are released. When the transmission or retransmission procedure is extended to a UM DRB, the PDCP layer connected to an RLC layer operating in a UM mode may regard data that is not transmitted to a lower layer yet, data for which PDCP discard timer is not expired, or a plurality of items of data to which PDCP SNs (or COUNT values) are already allocated, as data received or newly received from an upper layer. The PDCP layer may not restart a PDCP discard timer with respect to each of the data, may perform header (or data) compression, ciphering, or an integrity protection procedure on the data by using header (or data) compression context or a security key for the target BS, may generate and combine a PDCP header with the data, and may perform transmission or retransmission to a lower layer (e.g., an RLC layer of the second bearer for the target BS). The PDCP layer may process data in an ascending order of COUNT values allocated before the procedure is triggered, and may perform transmission or retransmission. The PDCP layer connected to the UM DRB or the AM DRB may not initialize a window state variable, but may maintain and use the window state variable.

When the buffer stores data to be transmitted, the PDCP layer may indicate, to the MAC layer of the second bearer for the target BS, that there is the data to be transmitted by indicating a size or amount (e.g., a PDCP data volume) of the data to be transmitted, and may perform switching of UL data transmission to the target BS. The MAC layer of the second bearer for the target BS may perform a scheduling request or a buffer status report procedure to receive allocation of a UL transmit resource from the target BS.

When receiving an indicator indicating that the first condition is satisfied from an upper layer (e.g., an RRC layer) or a lower layer (e.g., a MAC layer), an LTE or NR PDCP layer connected to the AM DRB (e.g., an RLC layer operating in an AM mode) may switch UL data transmission to a second bearer (e.g., an RLC layer or an MAC layer) for the target BS, and may transmit, to a lower layer (e.g., an RLC layer or a MAC layer) of a first bearer for the source BS, an indicator indicating to discard the data (e.g., a PDCP data (PDCP PDU)). This procedure is possible because the LTE or NR PDCP layer switches UL data transmission for the AM DRB and performs retransmission on the data for which successful delivery is not acknowledged, via the second bearer for the target BS, and thus, when data transmission to the source BS via the first bearer continues, transmission resources are wasted and unnecessary transmission is performed.

When the LTE or NR PDCP layer transmits, to the lower layer, the indicator indicating to discard the data, the LTE or NR PDCP layer may transmit a discard indicator only for PDCP user data (e.g., a PDCP data PDU) and may not transmit the discard indicator for PDCP control data (e.g., a PDCP control PDU such as a PDCP status report or ROHC feedback). This procedure is possible because the PDCP control data is information that is to be transmitted in order to continuously receive DL data from the source BS, even when the UE satisfies the first condition. When the PDCP control data is lost, an error may occur in the reception of the DL data. The LTE or NR PDCP layer connected to the AM DRB may switch UL data transmission to the second bearer (e.g., an RLC layer or an MAC layer) for the target BS, and may transmit, to the lower layer (e.g., an RLC layer or a MAC layer) of the first bearer for the source BS, an indicator indicating to discard all of the stored or possessed PDCP user data (e.g., PDCP data PDU) except for the PDCP control data (e.g., PDCP control PDU, e.g., a PDCP status report or ROHC feedback).

As another method, the LTE or NR PDCP layer connected to the AM DRB may switch UL data transmission to the second bearer (e.g., an RLC layer or an MAC layer) for the target BS, and may transmit, to the lower layer (e.g., an RLC layer or a MAC layer) of the first bearer for the source BS, an indicator indicating to discard stored or possessed PDCP user data or PDCP user data for which successful delivery is not acknowledged, except for the PDCP control data (PDCP control PDU, e.g., a PDCP status report or ROHC feedback). The NR PDCP layer may be connected to an LTE RLC layer or an NR RLC layer to perform data transmission or reception thereto or therefrom, and the LTE PDCP layer may be connected to only the LTE RLC layer and perform data transmission or reception, and thus, detailed descriptions thereof are proposed as follows.

When an RLC layer has received, from the LTE or NR PDCP layer, an indicator indicating to discard data (e.g., PDCP user data), and the RLC layer is an LTE RLC layer, the LTE RLC layer discards user data (e.g., a PDCP PDU, a PDCP Data PDU, or an RLC SDU) received from an upper layer (e.g., a PDCP layer) if a portion of the user data has not yet been mapped with RLC user data (e.g., an RLC data PDU) or has not been generated as RLC user data. When indicated from upper layer (i.e. PDCP) to discard a particular RLC SDU, the transmitting side of an AM RLC entity or the transmitting UM RLC entity shall discard the indicated RLC SDU if no segment of the RLC SDU has been mapped to an RLC data PDU yet. Therefore, if a portion of the user data has already been mapped with the RLC user data or has been generated as the RLC user data, the LTE RLC layer may not discard the user data and may perform data transmission to the source BS.

When an RLC layer has received, from the LTE or NR PDCP layer, an indicator indicating to discard data (e.g., a PDCP user data), and the RLC layer is an NR RLC layer, the NR RLC layer discards user data (e.g., a PDCP PDU, a PDCP Data PDU, or an RLC SDU) received from an upper layer (e.g., a PDCP layer) if the user data or a portion of the user data has not been delivered to a lower layer or has not been transmitted to the lower layer. When indicated from upper layer (i.e. PDCP) to discard a particular RLC SDU, the transmitting side of an AM RLC entity or the transmitting UM RLC entity shall discard the indicated RLC SDU, if neither the RLC SDU nor a segment thereof has been submitted to the lower layers. Therefore, if the user data or a portion of the user data has been delivered to a lower layer or has been transmitted to the lower layer, the NR RLC layer may not discard the user data and may perform data transmission to the source BS. Accordingly, the NR RLC layer, in contrast with the LTE RLC layer, may discard the user data if the user data has not been delivered to the lower layer, even when the user data has been generated as the RLC user data, and thus may discard more data and may more efficiently prevent unnecessary data transmission.

A procedure in which the LTE or NR PDCP layer connected to the AM DRB transmits a discard indicator to the lower layer may not be applied to the UM DRB in order to prevent transmission delay or data loss, because a re-transmission procedure may not be performed in the UM DRB.

However, as another method, after the LTE or NR PDCP layer connected to the AM DRB switches UL data transmission, a procedure in which the LTE or NR PDCP layer connected to the AM DRB transmits a discard indicator to the lower layer may be extended and applied to the UM DRB in order to minimize UL data transmission to the source BS. For example, when receiving an indicator indicating that the first condition is satisfied from an upper layer (e.g., an RRC layer) or a lower layer (e.g., a MAC layer), an LTE or NR PDCP layer connected to the UM DRB (e.g., an RLC layer operating in an UM mode) may switch UL data transmission to a second bearer (e.g., an RLC layer or an MAC layer) for the target BS, and may transmit, to a lower layer (e.g., an RLC layer or a MAC layer) of a first bearer for the source BS, an indicator to discard the data (e.g., PDCP data (PDCP PDU)). This procedure is possible because, when the LTE or NR PDCP layer switches UL data transmission with respect to the UM DRB and minimizes data transmission to the source BS via the first bearer, the LTE or NR PDCP layer can prevent transmission delay due to data forwarding from the source BS to the target BS and may prevent transmission resource waste of the source BS. When the LTE or NR PDCP layer transmits, to the lower layer, the indicator indicating to discard the data, the LTE or NR PDCP layer may transmit a discard indicator only for PDCP user data and may not transmit the discard indicator for PDCP control data. This procedure is possible because the PDCP control data is information that is to be transmitted in order to continuously receive DL data from the source BS, even when the UE satisfies the first condition. When the PDCP control data is lost, an error may occur in the reception of the DL data.

The LTE or NR PDCP layer connected to the UM DRB (e.g., an RLC layer operating in an UM mode) may switch UL data transmission to the second bearer (e.g., an RLC layer or an MAC layer) for the target BS, and may transmit, to the lower layer (e.g., an RLC layer or a MAC layer) of the first bearer for the source BS, an indicator indicating to discard all of the stored or possessed PDCP user data except for the PDCP control data.

As another method, the LTE or NR PDCP layer connected to the UM DRB may switch UL data transmission to the second bearer (e.g., an RLC layer or an MAC layer) for the target BS, and may transmit, to the lower layer (e.g., an RLC layer or a MAC layer) of the first bearer for the source BS, an indicator indicating to discard PDCP user data (e.g., PDCP data PDU) delivered by the PDCP layer to the lower layer or not actually transmitted by the lower layer, PDCP user data for which PDCP discard timer is not expired, or stored or possessed PDCP user data, except for the PDCP control data (PDCP control PDU, e.g., a PDCP status report or ROHC feedback). The NR PDCP layer may be connected to an LTE RLC layer or an NR RLC layer to perform data transmission or reception thereto or therefrom, and the LTE PDCP layer may be connected to only the LTE RLC layer and perform data transmission or reception, as described below.

When an RLC layer has received, from the LTE or NR PDCP layer, an indication to discard data (e.g., PDCP user data), and the RLC layer is an LTE RLC layer, the LTE RLC layer discards user data (e.g., PDCP PDU or PDCP Data PDU or RLC SDU) received from an upper layer (e.g., PDCP layer) if a portion of the user data has not yet been mapped with RLC user data (e.g., RLC data PDU) or has not been generated as RLC user data. When indicated from upper layer (i.e. PDCP) to discard a particular RLC SDU, the transmitting side of an AM RLC entity or the transmitting UM RLC entity shall discard the indicated RLC SDU if no segment of the RLC SDU has been mapped to an RLC data PDU yet. Therefore, if a portion of the user data has already been mapped with the RLC user data or has been generated as the RLC user data, the LTE RLC layer may not discard the user data and may perform data transmission to the source BS.

When an RLC layer has received, from the LTE or NR PDCP layer, an indication to discard data (e.g., PDCP user data), and the RLC layer is an NR RLC layer, the NR RLC layer discards user data (e.g., PDCP PDU or PDCP Data PDU or RLC SDU) received from an upper layer (e.g., PDCP layer) if the user data or a portion of the user data has not been delivered to a lower layer or has not been transmitted to the lower layer. When indicated from upper layer to discard a particular RLC SDU, the transmitting side of an AM RLC entity or the transmitting UM RLC entity shall discard the indicated RLC SDU, if neither the RLC SDU nor a segment thereof has been submitted to the lower layers. Therefore, if the user data or a portion of the user data has been delivered to a lower layer or has been transmitted to the lower layer, the NR RLC layer may not discard the user data and may perform data transmission to the source BS. Accordingly, the NR RLC layer, in contrast with the LTE RLC layer, may discard the user data if the user data has not been delivered to the lower layer even when the user data has been generated as the RLC user data, and thus may discard more data and may more efficiently prevent unnecessary data transmission.

According to Embodiment 2, even after the UE receives a handover command message (e.g., an RRCReconfiguration message), the UE may continuously receive DL data from the source BS or the target BS via the protocol layers of the first bearer for the source BS or the second bearer for the target BS. Further, to allow the UE to smoothly receive DL data from the source BS (or the target BS) or to allow the source BS (or the target BS) to smoothly transmit DL data to the UE, for AM bearers, the UE may be allowed to continuously perform UL transmission of an RLC status report, not data, on the source BS (or the target BS) via the protocol layers of the first bearer (or the second bearer). That is, even when the first condition is satisfied and the UE switches UL data transmission to the target BS, when the UE has to transmit the RLC status report, HARQ ACK or NACK, or PDCP control data (e.g., a PDCP ROHC feedback or a PDCP status report) to the source BS, the UE may be allowed to perform data transmission via the first bearer for the source BS. This procedure is possible because, in a case of the AM bearers, when data is transmitted to a transmitting terminal and then successful delivery is not indicated by using an RLC status report (i.e., when the RLC status report is not received), data cannot be continuously transmitted thereafter. Even when the first condition is satisfied in third operation 1 h-03 of FIG. 1H, the UE 1 h-20 discontinues UL data transmission to the source BS 1 h-05 via the protocol layers 1 h-22 of the first bearer, performs switching, and then starts UL data transmission to the target BS 1 h-10 via the protocol layers 1 h-21 of the second bearer, the UE 1 h-20 may continuously transmit HARQ ACK or HARQ NACK information, an RLC status report (ACK or NACK information), or PDCP control data via the protocol layers of the first bearer (or the second bearer) in order to smoothly receive DL data from the source BS 1 h-05 (or the target BS 1 h-10) or to allow the source BS 1 h-05 (or the target BS 1 h-10) to smoothly transmit DL data.

In the third operation 1 h-03, in accordance with Embodiment 2, even when the first condition is satisfied and the UE 1 h-20 discontinues UL data transmission to the source BS 1 h-05 via the protocol layers 1 h-22 of the first bearer, performs switching, and then starts UL data transmission to the target BS 1 h-10 via the protocol layers 1 h-21 of the second bearer, the UE 1 h-20 may continuously perform data transmission due to HARQ retransmission by the MAC layer or data transmission due to retransmission by the RLC layer in the AM mode in order to prevent loss of data to the source BS 1 h-05.

In the third operation 1 h-03, when the first condition is satisfied and the UE 1 h-20 discontinues UL data transmission to the source BS 1 h-05 via the protocol layers 1 h-22 of the first bearer, performs switching, and then starts UL data transmission to the target BS 1 h-10 via the protocol layers 1 h-21 of the second bearer, the source BS 1 h-05 or the target BS 1 h-10 may divide a time and may allocate a transmit resource to the UE 1 h-20 in order to prevent collision between a UL transmit resource to the target BS 1 h-10 and a UL transmit resource to the source BS 1 h-05. When the UL transmit resource to the target BS 1 h-10 collides with and overlaps with the UL transmit resource to the source BS 1 h-05, the UE 1 h-20 may perform data transmission to the source BS 1 h-05 by giving priority to the UL transmit resource to the source BS 1 h-05 in order to maintain transmission of DL data or continuously receive the DL data from the source BS 1 h-05 without a problem.

Alternatively, when the UL transmit resource to the target BS 1 h-10 collides with and overlaps with the UL transmit resource to the source BS 1 h-05, the UE 1 h-20 may perform data transmission to the target BS 1 h-10 by giving priority to the UL transmit resource to the target BS in order to maintain transmission of DL data from the target BS 1 h-10.

When the UE receives a handover command message in which handover of Embodiment 2 is indicated or is indicated for each bearer, the UE 1 h-20 or a bearer for which the DAPS handover is indicated may perform a scheduling request via a first protocol layer, may receive a UL transmit resource by transmitting a buffer status report to the source BS 1 h-05, may transmit UL data, and may receive DL data from the source BS 1 h-05 until the first condition is satisfied. However, when the first condition is satisfied, the UE 1 h-20 does not transmit data to the source BS anymore, may perform a scheduling request via a second protocol layer by switching a UL, may receive a UL transmit resource by transmitting a buffer status report to the target BS 1 h-10, and may transmit UL data to the target BS 1 h-10.

According to an embodiment, the UE 1 h-20 may continuously receive DL data from the source BS 1 h-05, and, even after UL transmission switching, the UE 1 h-20 may continuously transmit HARQ ACK or HARQ NACK, an RLC status report, or PDCP control data (e.g., a PDCP status report or ROHC feedback information) which corresponds to the DL data. The UE 1 h-20 may continuously receive DL data from the source BS 1 h-05 or the target BS 1 h-10 even when the first condition is satisfied.

When a second condition is satisfied in fourth operation 1 h-04 of FIG. 1H, the UE 1 h-20 may discontinue DL data reception from the source BS 1 h-05 via the protocol layers 1 h-22 of the first bearer or may release connection to the source BS 1 h-05.

The second condition may be one of the conditions described below. The PDCP layer 1 h-21 of the second bearer may continuously perform data transmission or reception without interruption to or from the target BS by using data to be transmitted or data to be received, SN information, or header compression and decompression context, which is stored in the PDCP layer 1 h-22 of the first bearer.

When the UE 1 h-20 performs a random access procedure on the target BS via layers 1 h-21 of the second bearer and receives a RAR message, it may be determined that the second condition is satisfied.

When the UE 1 h-20 performs a random access procedure on the target BS via the layers 1 h-21 of the second bearer, receives a RAR message, and configures and transmits a handover complete message to the target BS, it may be determined that the second condition is satisfied.

When the UE 1 h-20 performs a random access procedure on the target BS via the layers 1 h-21 of the second bearer, and first transmits data by using a PUCCH or PUSCH UL transmit resource or first receives the PUCCH or PUSCH UL transmit resource, it may be determined that the second condition is satisfied.

When a BS configures a separate timer to a UE by using an RRC message and the separate timer has expired, it may be determined that the second condition is satisfied.

The separate timer may start when the UE receives a handover command message from a source BS, the UE starts a random access to a target BS (e.g., transmits a preamble), the UE receives a RAR message from the target BS, the UE transmits a handover complete message to the target BS, or the UE first transmits data by using a PUCCH or PUSCH UL transmit resource.

When the UE performs a random access procedure on the target BS via protocol layers of a second bearer, receives a RAR message, configures and transmits a handover complete message to the target BS, and then receives acknowledgement with respect to successful delivery (HARQ ACK) of the handover complete message via a MAC layer or an RLC layer (RLC ACK), it may be determined that the second condition is satisfied.

When the UE performs a random access procedure on the target BS via the protocol layers of the second bearer, receives a RAR message or configures and transmits a handover complete message to the target BS, and then first receives allocation of a UL transmit resource from the target BS or first receives an indication of the UL transmit resource, it may be determined that the second condition is satisfied.

When the source BS performs efficient handover proposed in the disclosure, the source BS may determine when to discontinue transmission of DL data to the UE or when to release connection to the UE. For example, the source BS may determine whether to discontinue transmission of DL data or when to release connection to the UE, according to a certain method (e.g., when a certain timer expires after a handover is indicated) or the source BS receives, from the target BS, an indication indicating that the UE has successfully performed handover to the target BS). When the UE does not receive DL data from the source BS for a certain time period, the UE may determine that the second condition is satisfied, and may determine that connection to the source BS is released and may release the connection.

When the UE receives, from the target BS, an indication to release a connection to the source BS, or receives, from the target BS, a MAC CE, an RLC control PDU, or a PDCP control PDU, the UE may determine that the second condition is satisfied.

When the UE receives, from the source BS, an indication to release a connection to the source BS, or receives, from the target BS, a MAC CE, an RLC control PDU, or a PDCP control PDU, the UE may determine that the second condition is satisfied.

When the UE does not receive DL data from the source BS for a certain time period, the UE may determine that the second condition is satisfied.

When the UE successfully completes a random access procedure on the target BS via the layers of the second bearer and then receives allocation of a first UL transmit resource from the target BS or first receives an indication of a UL transmit resource, it may be determined that the second condition is satisfied.

For example, when the UE receives a handover command message from the source BS and receives an indication of a random access to the target BS, when the indicated random access is a CFRA (e.g., when a predefined preamble or a UE-cell identifier (e.g., C-RNTI is allocated), it may be determined that the random access procedure is successfully completed when the UE transmits the predefined preamble to a cell of the target BS and receives a RAR message. Therefore, when the UE receives a first UL transmit resource allocated, or included, or indicated in the RAR message, it may be determined that the second condition is satisfied.

As another method, when the UE first receives a UL transmit resource after the UE receives the RAR message, it may be determined that the second condition is satisfied.

When the UE receives a handover command message from the source BS and receives an indication of a random access to the target BS, when the indicated random access is a CBRA (e.g., when a predefined preamble ora UE-cell identifier (e.g., C-RNTI) is not allocated), it may be determined that the random access procedure on the target BS is successfully completed when the UE transmits a preamble to a cell of the target BS and receives a RAR message, transmits a message3 (e.g., a handover complete message) to the target BS by using a UL transmit resource allocated, included, or indicated in the RAR message, and receives, from the target BS and via a message4, a contention resolution MAC CE indicating resolution of contention or receives a UL transmit resource via a PDCCH corresponding to a C-RNTI of the UE. Therefore, when the UE monitors the PDCCH and first receives or is first indicated with the UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that the second condition is satisfied.

As another method, when a size of the UL transmit resource allocated in the RAR message is sufficient and thus the UE can transmit the message3 and additionally transmit UL data, the UE may determine that the UE first receives a UL transmit resource and thus may determine that the second condition is satisfied. In other words, when the UE receives a RAR message, the UE may determine that the UE first receives the UL transmit resource and thus may determine that the second condition is satisfied.

When a handover method (e.g., a RACH-less handover) that does not require a random access procedure is also indicated in the handover command message received by the UE, and when the handover command message includes a UL transmit resource with respect to the target BS, the UE transmits a message3 (e.g., a handover complete message or an RRCReconfigurationComplete message) by using the UL transmit resource with respect to the target BS, and when the UE receives, from the target BS, a UE Identity Confirmation MAC CE via a message4, or receives a UL transmit resource via a PDCCH corresponding to a C-RNTI of the UE, it may be determined that a random access procedure is successfully completed and the second condition is satisfied.

As another method, when the random access procedure is successfully completed and the UE performs PDCCH monitoring and receives a first UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that the second condition is satisfied.

When the handover command message does not include the UL transmit resources with respect to the target BS, the UE performs PDCCH monitoring on the target BS (or a cell) and when the UE receives a UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, or transmits a message3 (e.g., a handover complete message or an RRCReconfigurationComplete message) by using the UL transmit resource, and receives a UE Identity Confirmation MAC CE from the target BS or receives a UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that a random access procedure is successfully completed and the second condition is satisfied.

As another method, when the random access procedure is successfully completed and the UE performs PDCCH monitoring and receives a first UL transmit resource via the PDCCH corresponding to the C-RNTI of the UE, it may be determined that the second condition is satisfied.

When the UE performs Embodiment 2, when it is identified that the RRC layer, the MAC layer, or the RLC layer of the first bearer of the UE for the source BS, and the RRC layer, the MAC layer, or the RLC layer of the second bearer of the UE for the target BS satisfy the second condition proposed in the disclosure, an indicator indicating that the second condition is satisfied may be provided to a PDCP layer of the UE or a bearer that performs the DAPS handover method. When the PDCP layer of the UE receives, from a lower layer or an upper layer, the indication that the second condition is satisfied, the UE may perform one or more procedures below, thereby performing Embodiment 2.

The UE may release the first bearer for the source BS and may release a connection to the source BS.

When the UE releases a connection to the source BS, in order to report, to the target BS, a reception status of a plurality of items of DL data received from the source BS, the UE may trigger a PDCP status report procedure, may configure a PDCP status report, and may transmit the PDCP status report to the target BS.

When the second condition is satisfied, the UE may switch a second PDCP layer architecture or function (e.g., a second PDCP layer architecture or function 1 i-20 of FIG. 1I) to a first PDCP layer architecture or function (e.g., a first PDCP layer architecture or function 1 i-11 or 1 i-12 of FIG. 1I) with respect to each bearer or a bearer for which the DAPS handover method is indicated. The UE may initialize a variable for realignment, may stop and reset the realignment timer, may perform a deciphering procedure or header (or data) decompression by applying a security key or header decompression context for the source BS to a plurality of items of data (e.g., a plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS. The UE may provide the plurality of items of processed data to the upper layer in ascending order. That is, when the second condition is satisfied, the UE may perform the deciphering procedure or header (or data) decompression by applying the security key or header decompression context for the source BS to the plurality of items of data (e.g., the plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS.

As another method, when the second condition is satisfied, the UE may switch the second PDCP layer architecture or function to a third PDCP layer architecture or function (e.g., a third PDCP layer architecture or function 1 i-30 of FIG. 1I) with respect to each bearer or a bearer for which the DAPS handover method is indicated. The UE also may not stop nor initialize but may continuously use the variable for realignment and the realignment timer. However, the UE may perform the deciphering procedure or header (or data) decompression by applying the security key or header decompression context for the source BS to the plurality of items of data (e.g., the plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS. The UE may provide the plurality of items of processed data to the upper layer in an ascending order. That is, when the second condition is satisfied, the UE may perform the deciphering procedure or header (or data) decompression by applying the security key or header decompression context for the source BS to the plurality of items of data (e.g., the plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS. The UE may release QoS mapping information of the SDAP layer for the source BS, security key information of the PDCP layer for the source BS, header (or data) compression context information for the source BS, or the RLC layer or the MAC layer for the source BS.

For example, referring again to FIG. 1f , when the source gNB 1 f-02 transmits the handover command message to the UE 1 f-01 in step 1 f-20, the source gNB 1 f-02 may define indicators related to embodiments in the handover command message (e.g., an RRCReconfiguration message), and may indicate, to the UE 1 f-01, which handover procedure corresponding to which embodiment is to be triggered. The UE 1 f-01 may perform a handover procedure according to a handover method indicated in the handover command message. For example, the UE 1 f-01 may perform handover to the target gNB 1 f-03 in a manner that the UE 1 f-01 minimizes a data interruption time by performing the efficient handover method of Embodiment 2.

As another method, the source gNB 1 f-02 may define indicators for respective bearers, the indicators being related to embodiments, in the handover command message, and may further particularly indicate which embodiment is to be applied to which bearer in handover. For example, the source gNB 1 f-02 may indicate, via the handover command message, to apply Embodiment 2 only to the AM bearer in which the RLC layer operating in the AM mode is active, or may extensively apply Embodiment 2 to the UM bearer in which the RLC layer operating in the UM mode is active.

It is assumed herein that embodiments are applied to a DRB. However, when required (e.g., when the UE fails to perform handover to the target BS while the UE maintains a SRB with respect to the source BS, and thus the UE can report a handover failure message via the SRB with respect to the source BS or can recover a connection to the source BS) embodiments may be applied to the SRB.

When the UE performs data transmission and reception to and from the source BS via the protocol layers of the first bearer and performs data transmission and reception to and from the target BS via the protocol layers of the second bearer, the MAC layer of the first bearer and the MAC layer of the second bearer may each operate a discontinuous reception (DRX) period, thereby reducing battery consumption in the UE. Even after the UE receives the handover command message, the UE may continuously apply the DRX period of the MAC layer that was applied when transmitting and receiving data via the protocol layers of the first bearer, and may discontinue the DRX period according to the first condition or the second condition. The UE may manage, in response to indication from the target BS, whether to separately apply the DRX period to the MAC layer of the second bearer.

The UE discontinuing UL transmission to the source BS via the protocol layers of the first bearer and discontinuing DL data reception from the source BS may necessitate that the UE re-establishes, initializes, or releases the protocol layers (e.g., the PHY layer, the MAC layer, the RLC layer, or the PDCP layer) of the first bearer.

For convenience of description, the UE is described as configuring the first bearer for the source BS or the second bearer for the target BS, and embodiments may be easily extended and equally applied to a case in which the UE configures a plurality of first bearers for the source BS or a plurality of second bearers for the target BS. Further, embodiments are also applicable to a scenario in which a plurality of bearers for a plurality of target BSs are configured. For example, the UE may configure second bearers while performing a handover procedure on a first target BS, and when handover fails, the UE configures second bearers while performing a handover procedure on a second target BS, such that the UE may autonomously detect and determine cells satisfying a certain condition (e.g., a signal whose strength being greater than or equal to a certain value) from among a plurality of cells, may select one cell and then may perform a handover procedure on the cell.

FIG. 1I illustrates architectures of an efficient PDCP layer, according to an embodiment.

Referring to FIG. 1I, various architectures and functions are provided for the efficient PDCP layer which are to be applied to the DAPS handover method of Embodiment 2, and when a DAPS handover procedure is performed, different PDCP layer architectures as the architectures of the PDCP layer to be proposed below may be applied to each bearer at different time points.

For example, before the UE receives a handover command message from a BS, the UE may process and transmit or receive data by applying the first PDCP layer architecture and functions 1 i-11 or 1 i-12 proposed in the disclosure to each bearer in operation 1 i-01.

However, when the UE receives a handover command message from the BS, and the DAPS handover method proposed in the disclosure is indicated in the handover command message or the DAPS handover method is indicated for particular bearers, the UE may process and transmit or receive data by applying the second PDCP layer architecture and function 1 i-20 with respect to each bearer or bearers for which the DAPS handover method is indicated in operation 1 i-02. In other words, when the UE receives the handover command message from the BS, and the DAPS handover method is indicated in the handover command message or the DAPS handover method is indicated for particular bearers, the UE may switch the first PDCP layer architecture or function 1 i-11 or 1 i-12, which is used for each bearer, to the second PDCP layer architecture or function 1 i-20 proposed in the disclosure with respect to each bearer or a bearer for which the DAPS handover method is indicated.

As another method, when the first condition proposed in the disclosure is satisfied, the UE may switch the first PDCP layer architecture or function 1 i-11 or 1 i-12, which is used for each bearer, to the second PDCP layer architecture or function 1 i-20 proposed in the disclosure with respect to each bearer or a bearer for which the DAPS handover method is indicated in operation 1 i-02. When the UE receives the handover command message from the BS, and the DAPS handover method is indicated in the handover command message, the DAPS handover method is indicated for particular bearers, or a PDCP realignment timer value is newly set, when the UE switches the first PDCP layer architecture or function 1 i-11 or 1 i-12 to the second PDCP layer architecture or function 1 i-20 proposed in the disclosure with respect to each bearer or a bearer for which the DAPS handover method is indicated, the UE may update a variable for realignment to a PDCP SN or a COUNT value, which is predicted to be received next, and may stop and restart a realignment timer.

When the second condition is satisfied and the UE performs the DAPS handover method, the UE may release, from first bearers for the source BS, the second PDCP layer architecture and function 1 i-20 applied to each bearer or a bearer for which the DAPS handover method is indicated, and may switch back to the first PDCP layer architecture and function 1 i-11 or 1 i-12 and may apply the first PDCP layer architecture and function 1 i-11 or 1 i-12 to each bearer. When the second condition is satisfied and the UE switches the second PDCP layer architecture or function 1 i-20 to the first PDCP layer architecture or function 1 i-11 or 1 i-12 with respect to each bearer or a bearer for which the DAPS handover method is indicated, the UE may initialize a variable for realignment, may stop and reset the realignment timer, may perform a deciphering procedure or header (or data) decompression by applying a security key or header decompression context for the source BS to a plurality of items of data (e.g., a plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS. The UE may provide the plurality of items of processed data to the upper layer in an ascending order. In other words, when the second condition is satisfied, the UE may perform the deciphering procedure or header (or data) decompression by applying the security key or header decompression context for the source BS to the plurality of items of data (e.g., the plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS.

As another method, when the second condition is satisfied and the UE performs the DAPS handover method, the UE may release, from bearers for the source BS, the second PDCP layer architecture and function 1 i-20 applied to each bearer or a bearer for which the DAPS handover method is indicated, and may switch to the third PDCP layer architecture and function 1 i-30 and may apply the third PDCP layer architecture and function 1 i-30 to each bearer. When the second condition is satisfied and the UE switches the second PDCP layer architecture or function 1 i-20 to the third PDCP layer architecture or function 1 i-30 proposed in the disclosure with respect to each bearer or a bearer for which the DAPS handover method is indicated, the UE may not stop nor initialize but may continuously use the variable for realignment and the realignment timer. However, the UE may perform the deciphering procedure or header (or data) decompression by applying the security key or header decompression context for the source BS to the plurality of items of data (e.g., the plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS. The UE may provide the plurality of items of processed data to the upper layer in ascending order. That is, when the second condition is satisfied, the UE may perform the deciphering procedure or header (or data) decompression by applying the security key or header decompression context for the source BS to the plurality of items of data (e.g., the plurality of items of data received from the source BS) stored for reordering in the buffer, and may discard the security key or the header decompression context for the source BS.

As illustrated in FIG. 1I, the UE may apply, to each bearer, the first PDCP layer architecture or function 1 i-11 or 1 i-12, the second PDCP layer architecture or function 1 i-20, or the third PDCP layer architecture or function 1 i-30, which are different from each other, at different time points, such that data loss may be prevented and a data interruption time may be minimized when handover is performed.

The first PDCP layer architecture 1 i-11 or 1 i-12 in FIG. 1I may have a 1-1 PDCP layer architecture, a 1-2 PDCP layer architecture, a 1-3 PDCP layer architecture, or a 1-4 PDCP layer architecture, and may have characteristics to be described below.

For 1-1 PDCP layer architecture, e.g., when the UE applies the first PDCP layer architecture and function 1 i-11 to a PDCP layer (e.g., E-UTRA PDCP layer or LTE PDCP layer) connected to an AM RLC layer (e.g., E-UTRA AM RLC layer), the PDCP layer may have characteristics below.

The reception PDCP layer may first perform detection of out-of-window data or duplicate data on a plurality of items of received data. Retransmission may occur in RLC AM, and sizes of LTE RLC SN and PDCP SN may be different, such that the duplicate data or the out-of-window data may be received. In the above, window indicates a range of PDCP SNs or COUNT values, in which valid data is received.

Before the UE discards the out-of-window data or the duplicate data, the UE performs a deciphering procedure and a header decompression procedure and then performs a discard operation. Because the data may include useful information (e.g., initialization and refresh (IR) packet or header compression information) for the header decompression procedure, the UE may check and then discard the data.

The PDCP layer may immediately decipher a plurality of data items without ordering, the data being received without being discarded, and may perform a header decompression procedure. This is possible because the E-UTRA AM RLC layer performs ordering on the plurality of items of data and provides the plurality of items of data to the PDCP layer.

The PDCP layer provides the plurality of items of data to an upper layer in ascending order of COUNT values.

For 1-2 PDCP layer architecture, e.g., when the UE applies the first PDCP layer architecture and function 1 i-11 to a PDCP layer (e.g., E-UTRA PDCP layer or LTE PDCP layer) connected to a UM RLC layer (e.g., E-UTRA UM RLC layer), the PDCP layer may have characteristics below.

The PDCP layer may not perform a procedure of detecting out-of-window data or duplicate data. This is because the UM E-UTRA RLC layer does not perform a retransmission procedure.

The PDCP layer may immediately perform a deciphering procedure and then a header decompression procedure on the plurality of items of received data.

The PDCP layer may perform a reordering procedure and may provide the plurality of items of data to the upper layer (e.g., in an ascending order).

For 1-3 PDCP layer architecture, e.g., when the UE applies the first PDCP layer architecture 1 i-11 to the PDCP layer (e.g., the E-UTRA PDCP layer or the LTE PDCP layer) configured with a split bearer, a packet duplication bearer, or a LTE WLAN Aggregation (LWA) bearer, a reordering procedure and a realignment timer may always be applied and the PDCP layer may have characteristics below.

The PDCP layer may first perform detection of out-of-window data or duplicate data on a plurality of items of received data. Retransmission may occur in RLC AM, data may be received at different time points from different RLC layers, and sizes of LTE RLC SN and PDCP SN may be different, such that the out-of-window data or the duplicate data may be received.

The PDCP layer performs a deciphering procedure. However, the PDCP layer may not perform a header decompression procedure because the E-UTRA PDCP layer cannot configure a header compression protocol to the split bearer or the LWA bearer.

When an integrity protection or verification procedure has been performed, the PDCP layer may perform the integrity protection or verification procedure and then discard data. When the integrity verification procedure fails, the PDCP layer may discard the data and may report the failure to an upper layer.

The PDCP layer discards the out-of-window data or the duplicate data.

When the data is not discarded, the PDCP layer may immediately perform a deciphering procedure without reordering on a plurality of items of received data. Afterward, when the integrity protection or verification procedure is configured, the PDCP layer may perform integrity verification. When an integrity protection or verification procedure has been performed, the PDCP layer may perform the integrity protection or verification procedure and then discard data. When the integrity verification procedure fails, the PDCP layer may discard the data and may report the failure to an upper layer.

Afterward, the PDCP layer may perform reordering on a plurality of items of received data, and when PDCP SNs or COUNT values are sequentially aligned in an ascending order without a gap therebetween, the PDCP layer may perform a header compression procedure (e.g., when the header compression procedure or a header decompression procedure is configured) and may provide the data to the upper layer in the ascending order.

When a realignment timer is running and data is provided to the upper layer, the data corresponding to COUNT value having the same value as a value obtained by subtracting 1 from a value a variable for realignment maintains, or when the plurality of items of data are all provided to the upper layer without a gap between PDCP SNs (COUNT values), the PDCP layer stops and resets the realignment timer. When the realignment timer is not running and a buffer stores data that is not provided to the upper layer, or when there is a gap between PDCP SNs (COUNT values), the PDCP layer starts the realignment timer.

The PDCP layer updates the variable for realignment to a PDCP SN or a COUNT value which is predicted to be received next time.

When the realignment timer is expired and a header decompression procedure is configured to values of a plurality of items of stored data, the values being smaller than the variable for realignment, in ascending order of PDCP SNs or COUNT values, the PDCP layer performs the header decompression procedure and provides the data to the upper layer.

When a header decompression procedure is configured to values of a plurality of items of stored data, the values being equal to or greater than the variable for realignment, in an ascending order of PDCP SNs or COUNT values, the PDCP layer performs the header decompression procedure and provides the data to the upper layer.

The PDCP layer updates a variable value of data, which is most recently provided to the upper layer, to a PDCP SN or a COUNT value of the data most recently provided to the upper layer.

When a buffer stores data that is not provided to the upper layer, or when there is a gap between PDCP SNs (COUNT values), the PDCP layer starts the realignment timer.

The PDCP layer updates the variable for realignment to a PDCP SN or a COUNT value which is predicted to be received next time.

For 1-4 PDCP layer architecture, e.g., when the UE applies the first PDCP layer architecture and function 1 i-12 to an NR PDCP layer, the PDCP layer may always apply a reordering procedure and a realignment timer and may have characteristics below.

The PDCP layer may first perform a deciphering procedure on a plurality of items of received data.

When an integrity protection or verification procedure is configured, the PDCP layer may perform the integrity protection or verification procedure on the received data, and when the integrity verification procedure fails, the PDCP layer may discard the data and may report the failure to an upper layer.

The PDCP layer performs detection of out-of-window data or duplicate data on the received data. For example, the deciphering procedure is first performed and then the detection of out-of-window data or duplicate data is performed. As another method, the deciphering procedure may be performed only when the integrity protection or verification procedure is configured. When the detection of out-of-window data or duplicate data is performed but the integrity protection or verification procedure is not configured, the deciphering procedure may be performed only on a plurality of items of data on which the detection of out-of-window data or duplicate data is performed and that are not discarded.

The PDCP layer discards the out-of-window data or the duplicate data.

When the data is not discarded, the PDCP layer may perform reordering on a plurality of items of received data, and when PDCP SNs or COUNT values are sequentially aligned in an ascending order without a gap therebetween, the PDCP layer may perform a header compression procedure (e.g., when the header compression procedure or a header decompression procedure is configured) and may provide the data to the upper layer in ascending order.

The UE provides the plurality of items of data to an upper layer in ascending order of COUNT values.

When a realignment timer is running, and data is provided to the upper layer, the data corresponding to COUNT value having the same value as a value obtained by subtracting 1 from a value a variable for realignment maintains, when the plurality of items of data are all provided to the upper layer without a gap between PDCP SNs (COUNT values), or when a value of a variable storing a PDCP SN or a COUNT value of data to be provided to the upper layer is equal to or greater than a value of a variable for realignment, the PDCP layer stops and resets the realignment timer.

When the realignment timer is not running, and when a buffer stores data that is not provided to the upper layer, when there is a gap between PDCP SNs (COUNT values), or when a value of a variable storing a COUNT value of first data that is not provided to the upper layer is smaller than a value of a variable for realignment, the PDCP layer updates the variable for realignment to a PDCP SN or a COUNT value which is predicted to be received next time.

The PDCP layer starts the realignment timer.

When the realignment timer is expired, and a header decompression procedure is configured to values of a plurality of items of stored data, the values being smaller than the variable for realignment, in an ascending order of PDCP SNs or COUNT values, the PDCP layer performs the header decompression procedure and provides the data to the upper layer.

When a header decompression procedure is configured to values of a plurality of items of stored data, the values being equal to or greater than the variable for realignment, in an ascending order of PDCP SNs or COUNT values, the PDCP layer performs the header decompression procedure and provides the data to the upper layer.

The PDCP layer updates a variable value of first data, which is not provided to the upper layer, to a PDCP SN or a COUNT value of the first data that is not provided to the upper layer.

When a buffer stores data that is not provided to the upper layer, when there is a gap between PDCP SNs (COUNT values), or when a value of a variable storing a COUNT value of first data that is not provided to the upper layer is smaller than a value of a variable for realignment, the PDCP layer updates the variable for realignment to a PDCP SN or a COUNT value which is predicted to be received next time.

The PDCP layer starts the realignment timer.

The second PDCP layer architecture or function 1 i-20 in FIG. 1I may have a 2-1 PDCP layer architecture or a 2-2 PDCP layer architecture, and may have characteristics to be described below.

The second PDCP layer architecture 1 i-20, which is efficient in handover, may be applied to Embodiment 2 for minimizing a data interruption time.

In the second PDCP layer architecture, the UE may perform data transmission or reception from or to a source BS 1 i-21 via protocol layers (e.g., a SDAP layer, a PDCP layer, an RLC layer, or a MAC layer) of a first bearer, and may perform data transmission or reception from or to a target BS 1 i-22 via protocol layers of a second bearer.

The PDCP layer of the first bearer and the PDCP layer of the second bearer may each be configured in the UE but may logically operate as one PDCP layer as illustrated in 1 i-20. By distinguishing between functions of a PDCP layer, the one PDCP layer may be implemented as functions (e.g., an SN allocation function, a realignment function, an in-sequence delivery function, or a duplicate detection function) of an upper PDCP layer and functions (e.g., a deciphering or ciphering function, a header (or data) compression or decompression function, an integrity protection or verification function, or a duplicate detection function) of two lower PDCP layers respectively for the source BS and the target BS.

As described above, when the DAPS handover method is performed, the UE may transmit UL data transmission to the source BS, and when the first condition is satisfied, the UE may switch to the target BS and may continuously receive DL data from the source BS and the target BS. Therefore, only one header (or data) compression protocol context for the source BS or the target BS may be maintained and applied to a UL, and two contexts for the source BS or the target BS may be maintained and applied to a DL.

The 2-1 PDCP layer architecture (e.g., an E-UTRA PDCP layer for the DAPS handover method) proposed in the disclosure, based on the second PDCP layer architecture, may have characteristics below.

An upper transmit PDCP layer function may allocate PDCP SNs to a plurality of items of data received from an upper layer. Two lower transmit PDCP layer functions 1 i-21 and 1 i-22 for the source BS and the target BS, respectively, may apply, to data to be transmitted to the source BS, header (or data) compression context or security key configured with the source BS, by using a separate security key configured with each of the source BS and the target BS, may apply, to data to be transmitted to the target BS, header (or data) compression context or security key configured with the target BS, and may apply a header (or data) compression procedure when the header (or data) compression procedure is configured. When integrity protection is configured, the lower transmit PDCP layer functions 1 i-21 and 1 i-22 may apply a ciphering procedure by applying an integrity protection procedure to a PDCP header and data (e.g., a PDCP SDU), may provide the data to be transmitted to the source BS to a transmit RLC layer of the first bearer, and may provide the data to be transmitted to the target BS to a transmit RLC layer of the second bearer, thereby performing transmission.

In order to accelerate a data processing speed, the two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform parallel processing to perform header compression, integrity protection, or a ciphering procedure in parallel. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform the integrity protection or the ciphering procedure by using different security keys. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform compression, integrity protection, or a ciphering procedure on different data by applying different compression contexts, different security keys, or different security algorithms in a logically-one transmit PDCP layer.

A receive PDCP layer function, namely, the lower receive PDCP layer functions 1 i-21 and 1 i-22 for the source BS or the target BS, may each independently perform an out-of-window data detection or duplicate detection procedure on data respectively received from lower layers, in particular, a plurality of items of data received from two RLC layers for each of the source BS and the target BS, based on PDCP SNs or COUNT values.

As another method, for convenience of implementation, the receive PDCP layer function may perform the out-of-window data detection or duplicate detection procedure on all received data, without distinguishing between the RLC layers, based on PDCP SNs or COUNT values.

As another method, for more accurate duplicate detection, the receive PDCP layer function may perform the out-of-window data detection based on PDCP SNs or COUNT values on all received data, without distinguishing between the RLC layers, and may separately perform the duplicate detection procedure on a plurality of items of data received from each of the RLC layers.

As yet another method, when data received from different BSs are overlapped with each other, in order to prevent data loss for a header compression protocol, the receive PDCP layer function may perform the out-of-window data detection based on PDCP SNs or COUNT values on all received data without distinguishing between the RLC layers, and may perform the duplicate detection procedure on all data after a deciphering procedure, an integrity protection procedure, or a header (or data) decompression procedure is received with respect to data received from each of the RLC layers.

When a deciphering procedure is immediately applied to a plurality of items of received data by using separate header (or data) compression context or security key separately configured with the source BS and the target BS and integrity protection is configured, sub-functions of the receive PDCP layer may apply an integrity protection procedure to the PDCP header and the data (PDCP SDU).

In the 2-1 PDCP layer architecture, a header (or data) decompression procedure may be immediately performed, without reordering, on a plurality of items of data received from RLC layers of the first bearer for the source BS, and a header (or data) decompression procedure may be immediately performed, without reordering, on a plurality of items of data received from RLC layers of the second bearer for the target BS. To distinguish between the data received from the RLC layers of the first bearer for the source BS and the data received from the RLC layers of the second bearer for the target BS, an indicator is defined for each of the received data such that it is possible to identify whether the PDCP layer received data from the source BS or received data from the target BS.

As another method, a 1-bit indicator is defined in a PDCP header, a SDAP header, or an RLC header, such that it is possible to identify whether the PDCP layer received data from the source BS or received data from the target BS. The PDCP layer may perform the duplicate detection procedure based on a PDCP SN or a COUNT value (e.g., a procedure in which only one data (including pre-received data or data provided to the upper layer) is allocated for each PDCP SN or each COUNT value and the others are all discarded) on all of the data received from RLC layers of the first bearer for the source BS and the data received from RLC layers of the second bearer for the target BS, wherein the header (or data) compression procedure has been completed with respect to the data. The PDCP layer may perform a realignment procedure on all of the data received from RLC layers of the first bearer for the source BS and the data received from RLC layers of the second bearer for the target BS, in an ascending order, based on PDCP SNs or COUNT values, and may sequentially provide the data to the upper layer. Because one PDCP layer can receive data in any order from different BSs, e.g., from the first bearer or the second bearer, the PDCP layer may have to always perform the realignment procedure.

As described above, to accelerate a data processing speed, the two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform parallel processing to perform header compression, integrity protection, or a ciphering procedure in parallel, based on each PDCP SN or each COUNT value. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform the integrity protection, the ciphering procedure, or the header decompression procedure by using different header (or data) compression contexts or different security keys. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform the integrity protection, the ciphering procedure, or the decompression procedure on different data by applying different header (or data) compression contexts, different security keys, or different security algorithms in a logically-one transmit PDCP layer. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform out-of-sequence deciphering or integrity verification procedure on each of a plurality of items of data received without relation to the order of PDCP SNs or COUNT values.

When the one PDCP layer distinguishes layers of the first bearer from layers of the second bearer, the PDCP layer may distinguish the layers (or a first RLC layer) of the first bearer from the layers (or a second RLC layer) of the second bearer, by considering that the layer of the first bearer and the layer of the second bearer are connected to different MAC layers, that the layers have different logical channel identifiers or are different RLC layers connected to different MAC layers, or that the layers use different ciphering keys. Accordingly, the PDCP layer may perform a ciphering procedure or a deciphering procedure on UL data and DL data by using different security keys, and may compress or decompress the UL data and the DL data by using different compression protocol contexts.

The 2-2 PDCP layer architecture (e.g., an NR PDCP layer for the DAPS handover method), based on the second PDCP layer architecture, may have the characteristics described below.

An upper transmit PDCP layer function may allocate PDCP SNs to a plurality of items of data received from an upper layer. Two lower transmit PDCP layer functions 1 i-21 and 1 i-22 for the source BS and the target BS, respectively, may apply, to data to be transmitted to the source BS, header (or data) compression context or security key configured with the source BS, by using a separate security key configured with each of the source BS and the target BS, may apply, to data to be transmitted to the target BS, header (or data) compression context or security key configured with the target BS, and may apply a header (or data) compression procedure when the header (or data) compression procedure is configured. When integrity protection is configured, the lower transmit PDCP layer functions 1 i-21 and 1 i-22 may apply a ciphering procedure by applying an integrity protection procedure to a PDCP header and data (e.g., a PDCP SDU), may provide the data to be transmitted to the source BS to a transmit RLC layer of the first bearer, and may provide the data to be transmitted to the target BS to a transmit RLC layer of the second bearer, thereby performing transmission. In order to accelerate a data processing speed, the two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform parallel processing to perform header compression, integrity protection, or a ciphering procedure in parallel. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform the integrity protection or the ciphering procedure by using different security keys. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform compression, integrity protection, or a ciphering procedure on different data by applying different compression contexts, different security keys, or different security algorithms in a logically-one transmit PDCP layer.

A receive PDCP layer function, namely, the lower receive PDCP layer functions 1 i-21 and 1 i-22 for the source BS or the target BS, may each independently perform an out-of-window data detection or duplicate detection procedure on data respectively received from lower layers, in particular, a plurality of items of data received from two RLC layers for each of the source BS and the target BS, based on PDCP SNs or COUNT values.

As another method, for convenience of implementation, the receive PDCP layer function may perform the out-of-window data detection or duplicate detection procedure on all received data, without distinguishing between the RLC layers, based on PDCP SNs or COUNT values.

As another method, for more accurate duplicate detection, the receive PDCP layer function may perform the out-of-window data detection based on PDCP SNs or COUNT values on all received data, without distinguishing between the RLC layers, and may separately perform the duplicate detection procedure on a plurality of items of data received from each of the RLC layers.

As yet another method, when data received from different BSs are overlapped with each other, in order to prevent data loss for a header compression protocol, the receive PDCP layer function may perform the out-of-window data detection based on PDCP SNs or COUNT values on all received data without distinguishing between the RLC layers, and may perform the duplicate detection procedure on all data after a deciphering procedure, an integrity protection procedure, or a header (or data) decompression procedure is received with respect to data received from each of the RLC layers.

When a deciphering procedure is immediately applied to a plurality of items of received data by using separate header (or data) compression context or security key separately configured with the source BS and the target BS and integrity protection is configured, sub-functions of the receive PDCP layer may apply an integrity protection procedure to the PDCP header and the data (PDCP SDU).

In the 2-2 PDCP layer architecture, a reordering procedure may be performed on a plurality of items of data received from RLC layers of the first bearer for the source BS and a plurality of items of data received from RLC layers of the second bearer for the target BS, and then may perform a header (or data) decompression procedure on the plurality of items of data received from each BS (the source BS or the target BS) in an ascending order of PDCP SNs or COUNT values, by applying header (or data) compression context of each BS (the source BS or the target BS). To distinguish between the data received from the RLC layers of the first bearer for the source BS and the data received from the RLC layers of the second bearer for the target BS, an indicator is defined for each of the received data such that it is possible to identify whether the PDCP layer received data from the source BS or received data from the target BS.

As another method, a 1-bit indicator is defined in a PDCP header, a SDAP header, or an RLC header, such that it is possible to identify whether the PDCP layer received data from the source BS or received data from the target BS. The PDCP layer may perform the duplicate detection procedure based on a PDCP SN ora COUNT value (e.g., a procedure in which only one data (including pre-received data or data provided to the upper layer) is allocated for each PDCP SN or each COUNT value and the others are all discarded) on all of the data received from RLC layers of the first bearer for the source BS and the data received from RLC layers of the second bearer for the target BS, wherein the header (or data) compression procedure has been completed with respect to the data. The PDCP layer may sequentially provide, to the upper layer, all of the data received from RLC layers of the first bearer for the source BS and the data received from RLC layers of the second bearer for the target BS, in an ascending order, based on PDCP SNs or COUNT values. Because one PDCP layer can receive data in any order from different BSs, e.g., from the first bearer or the second bearer, the PDCP layer may have to always perform the realignment procedure.

As described above, to accelerate a data processing speed, the two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform parallel processing to perform header compression, integrity protection, or a ciphering procedure in parallel, based on each PDCP SN or each COUNT value. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform the integrity protection, the ciphering procedure, or the header decompression procedure by using different header (or data) compression contexts or different security keys. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform the integrity protection, the ciphering procedure, or the decompression procedure on different data by applying different header (or data) compression contexts, different security keys, or different security algorithms in a logically-one transmit PDCP layer. The two lower transmit PDCP layer functions 1 i-21 and 1 i-22 may perform out-of-sequence deciphering or integrity verification procedure on each of a plurality of items of data received without relation to the order of PDCP SNs or COUNT values.

When the one PDCP layer distinguishes layers of the first bearer from layers of the second bearer, the PDCP layer may distinguish the layers (or a first RLC layer) of the first bearer from the layers (or a second RLC layer) of the second bearer, by considering that the layer of the first bearer and the layer of the second bearer are connected to different MAC layers, that the layers have different logical channel identifiers or are different RLC layers connected to different MAC layers, or that the layers use different ciphering keys. Accordingly, the PDCP layer may perform a ciphering procedure or a deciphering procedure on UL data and DL data by using different security keys, and may compress or decompress the UL data and the DL data by using different compression protocol contexts.

The third PDCP layer architecture 1 i-30, which is efficient in handover, may be applied to Embodiment 2 for minimizing a data interruption time, which is proposed in the disclosure. A PDCP layer function in the third PDCP layer architecture may be equal to that in the second PDCP layer architecture. However, the third PDCP layer architecture may correspond to an architecture from which the first bearer for the source BS in the second PDCP layer architecture is released. The third PDCP layer architecture may have the same functions as those of the second PDCP layer architecture, but may have an architecture from which the first bearer (e.g., the SDAP layer, the PDCP layer, the RLC layer, or the MAC layer) for the source BS in the second PDCP layer architecture is released. Therefore, the third PDCP layer architecture may be characterized in that QoS mapping information of the SDAP layer for the source BS, security key information for the PDCP layer for the source BS, header (or data) compression context information for the source BS, or the RLC layer or the MAC layer for the source BS is released.

FIG. 1J is a flowchart illustrating operations of a UE, according to an embodiment.

Referring to FIG. 1J, the UE performs data transmission or reception to or from a source BS for each bearer via a first PDCP layer architecture in step 1 j-01.

In step 1 j-05, the UE receives a handover command message. When the handover command message indicates the DAPS handover method of Embodiment 2 or indicates the DAPS handover method for each bearer, the UE may switch the architecture of a target BS indicated in the handover command message to a second PDCP layer architecture for each bearer or bearers for which the DAPS handover method is indicated. Even when the UE configures and establishes protocol layers of a second bearer in step 1 j-10, and performs a random access procedure on the target BS via the established protocol layers in step 1 j-15, the UE may continuously perform data transmission or reception to or from the source BS via protocol layers of a first bearer in step 1 j-20.

When the terminal satisfies the first condition in step 1 j-25, the UE discontinues UL data transmission to the source BS via the protocol layers of the first bearer, transmits UL data to the target BS via the protocol layers of the second bearer by switching the UL data transmission, and continuously receives DL data from the source BS and the target BS via the protocol layers of the first and second bearers in step 1 j-30. A PDCP layer of the second bearer may continuously perform data transmission or reception without interruption to or from the target BS by using data to be transmitted or data to be received, SN information, or header compression and decompression context, which is stored in a PDCP layer of the first bearer.

However, when the terminal does not satisfy the first condition in step 1 j-25, the UE continuously checks the first condition while continuously performing an ongoing procedure in step 1 j-35.

When the terminal satisfies the second condition in step 1 j-40, the UE discontinues DL data reception from the source BS via the protocol layers of the first bearer in step 1 j-45. The PDCP layer of the second bearer may continuously perform data transmission or reception without interruption to or from the target BS by using data to be transmitted or data to be received, SN information, or header compression and decompression context, which is stored in the PDCP layer of the first bearer.

However, when the terminal does not satisfy the second condition in step 1 j-40, the UE continuously checks the second condition while performing an ongoing procedure in step 1 j-50.

According to an embodiment, a PDCP layer may perform different procedures according to types of handover indicated in a handover command message received by a UE.

When a handover indicated in the handover command message the UE receives from a source BS is the handover of Embodiment 1 (e.g., a normal handover method), the UE may perform a PDCP re-establishment procedure on the PDCP layer according to each bearer. The UE may initialize window state variables with respect to a SRB and may discard a plurality of items of stored data (PDCP SDU or PDCP PDU). The UE may initialize window state variables with respect to a UM DRB, may perform, in an ascending order of COUNT values, compression, ciphering, or integrity protection on a plurality of items of data that are not transmitted to a lower layer yet or for which a PDCP discard timer is not expired, based on header (or data) compression context or security key of the target BS, and then may transmit or retransmit the data. When a realignment timer is running, the UE may stop and reset the realignment timer, may process the plurality of items of received data (e.g., a PDCP SDU or a PDCP PDU) in order, and may provide the data to an upper layer. The UE does not initialize window state variables with respect to an AM DRB, may perform, based on header (or data) compression context or security key of the target BS, compression, ciphering, or integrity protection on data starting from first data (e.g., a PDCP SDU or a PDCP PDU) for which successful delivery is not acknowledged by a lower layer, in an ascending order of PDCP SNs or COUNT values, and then may transmit or retransmit the data.

When the handover indicated in the handover command message the UE receives from a source BS is handover of Embodiment 2 (or the handover is indicated in the handover command message for each bearer), a PDCP layer that has received the handover command message and for which the DAPS handover method is indicated may not perform a PDCP re-establishment procedure and may perform procedures below. For example, the UE may initialize window state variables with respect to an SRB (e.g., may skip variable initialization to perform fallback when DAPS handover fails) or may discard the stored data (e.g., a PDCP SDU or a PDCP PDU). The UE may not initialize window state variables with respect to an UM DRB, and may continuously perform, to or from the source BS, data transmission or reception on a plurality of items of data that are not transmitted to a lower layer yet or for which a PDCP discard timer is not expired. The UE may not initialize window state variables with respect to an AM DRB, and may continuously perform data transmission or reception to or from the source BS.

The UE may perform procedures on each bearer (or a bearer for which Embodiment 2 is indicated), based on the first condition being satisfied.

The UE may perform procedures on each bearer (or a bearer for which Embodiment 2 is indicated), based on the second condition being satisfied.

When the source BS indicates, to the UE, a handover to which embodiments proposed in the disclosure are applied, the source BS may start data forwarding to a target BS when a third condition below is satisfied. The third condition may indicate that one or a plurality of conditions from among the conditions below is satisfied.

When the UE receives, from the target BS, an indication that handover is successfully completed, the third condition may be satisfied.

When the source BS transmits a handover command message to the UE, the third condition may be satisfied.

When the source BS transmits a handover command message to the UE and acknowledges successful delivery (HARQ ACK or NACK or RLC ACK or NACK) of the handover command message, the third condition may be satisfied.

When the source BS receives, from the UE, an indication (e.g., an RRC message (e.g., an RRCReconfiguration message)) indicating that connection to the source BS is to be release, or receives a MAC CE, an RLC control PDU, or PDCP control PDU from the UE, the third condition may be satisfied.

When the source BS transmits a handover command message to the UE and drives a certain timer, and the timer has expired, the third condition may be satisfied.

When an acknowledgement (HARQ ACK or NACK or RLC ACK or NACK) with respect to successful delivery of DL data is not received from the UE for a certain time, the third condition may be satisfied.

In the disclosure, provided is a method in which, when a BS indicates the Embodiment 1 (a normal handover method) or Embodiment 2 (DAPS handover method) to a UE through an RRC message (e.g., a handover command message) or indicates the Embodiment 1 or 2 for each bearer (or for each logical channel), the UE performs a handover procedure according to Embodiment 1 or 2, and the LTE or NR PDCP layer connected to the AM DRB (e.g., an RLC layer operating in an AM mode) or the LTE or NR PDCP layer connected to the UM DRB (e.g., an RLC layer operating in an UM mode) generates and configures a condition of triggering a PDCP status report and a triggered PDCP status report.

When a PDCP layer for each bearer satisfies one of the following conditions, the PDCP layer may trigger, generate, and configure the PDCP status report and transmit the PDCP status report to a lower layer, thereby performing transmission. The following operations may be applied to a PDCP layer of a UE or a BS.

When the LTE or NR PDCP layer connected to the AM DRB or to the RLC layer operating in an AM mode is set to trigger or transmit the PDCP status report by an upper layer (e.g., RRC layer) (or set whether to trigger the PDCP status report through an indicator (e.g., status Report Required) included in an RRC message), if the UE receives an RRC message (e.g., a handover message) from the BS and the RRC message indicates the handover method corresponding to Embodiment 1 to the UE and indicates a PDCP re-establishment procedure to the LTE or NR PDCP layer connected to the AM DRB or if an upper layer (e.g., an RRC layer) of the UE indicates the PDCP re-establishment procedure to the LTE or NR PDCP layer connected to the AM DRB, the LTE or NR PDCP layer connected to the AM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS.

If the UE receives an RRC message (e.g., a handover message) from the BS and the RRC message indicates the handover method corresponding to the first embodiment and indicates a PDCP data recovery procedure to the LTE or NR PDCP layer connected to the AM DRB or if an upper layer (e.g., an RRC layer) of the UE indicates the PDCP data recovery procedure to the LTE or NR PDCP layer connected to the AM DRB, the LTE or NR PDCP layer connected to the AM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS.

If the RRC message (e.g., a handover message) received by the UE from the BS indicates the handover method corresponding to Embodiment 2 to the UE or for each bearer of the UE and indicates a DAPS handover method (or through an indicator) to the LTE or NR PDCP layer connected to the AM DRB or if an upper layer (e.g., an RRC layer) of the UE indicates the DAPS handover method (or through an indicator) to the LTE or NR PDCP layer connected to the AM DRB, the LTE or NR PDCP layer connected to the AM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS. When the DAPS handover method is indicated as described above, the source BS may transmit a handover command message to the UE with respect to the data of an indicated bearer and may immediately start DL or UL data forwarding to the target BS. Thus, when the UE transmits the PDCP status report (e.g., an indication of whether DL data has been successfully received) to the source BS with respect to the DL data, unnecessary data forwarding may be prevented.

If the RRC message (e.g., a handover message) received by the UE, from the BS, indicates the handover method corresponding to the Embodiment 2 (DAPS handover method) to the UE or for each bearer of the UE, sets the DAPS handover method (or through an indicator) to the LTE or NR PDCP layer connected to the AM DRB, and a first condition proposed as described above is satisfied, and thus, a lower layer (e.g., MAC layer) or an upper layer (e.g., RRC layer when the first timer is stopped) transmits an indicator (e.g., an indicator indicating UL data transmission switching) to the PDCP layer and the PDCP layer receives the indicator or if the first condition proposed in the disclosure is satisfied, and thus a lower layer (e.g., MAC layer) or an upper layer (e.g., RRC layer when the first timer is stopped) transmits the indicator to the PDCP layer and the PDCP layer receives the indicator, then the LTE or NR PDCP layer connected to the AM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS. When the PDCP layer switches UL data transmission, the PDCP layer may transmit a PDCP status report (or new PDCP control data) to the source BS or the target BS, and thus may indicate, to the source BS or the target BS, that a PDCP layer corresponding to the bearer of the UE has switched UL data transmission. Because the source BS or the target BS may identify a time point at which the UE satisfies the first condition, the source BS or the target BS may manage transmission resources of the UE by reflecting the identified time point in scheduling.

If the RRC message (e.g., a handover message) received by the UE from the BS indicates the handover method corresponding to Embodiment 2 (DAPS handover method) to the UE or for each bearer of the UE, sets the DAPS handover method (or a procedure proposed in the disclosure through an indicator) to the LTE or NR PDCP layer connected to the AM DRB, and a second condition as described above is satisfied, and thus a lower layer (e.g., MAC layer) or an upper layer (e.g., RRC layer) transmits an indicator (e.g., an indicator indicating that connection to the source BS is to be released) to the PDCP layer and the PDCP layer receives the indicator or if the second condition proposed in the disclosure is satisfied, and thus a lower layer (e.g., MAC layer) or an upper layer (e.g., RRC layer) transmits an indicator (e.g., an indicator indicating that connection to the source BS is to be released) to the PDCP layer and the PDCP layer receives the indicator, then the LTE or NR PDCP layer connected to the AM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS. When the LTE or NR PDCP layer releases the source BS, the LTE or NR PDCP layer may transmit a PDCP status report (or new PDCP control data) to the source BS or the target BS, and thus may indicate, to the source BS or the target BS, that the UE has released connection to the source BS. Because the source BS or the target BS may identify a time point at which the UE satisfies the second condition, the source BS or the target BS may manage transmission resources of the UE by reflecting the identified time point in scheduling.

When the LTE or NR PDCP layer connected to the UM DRB or to the RLC layer operating in a UM mode is set to trigger or transmit the PDCP status report by an upper layer (e.g., RRC layer) (or set whether to trigger the PDCP status report through an indicator (e.g., status Report Required) included in an RRC message), if the RRC message (e.g., a handover message) received by the UE from the BS indicates the handover method corresponding to the Embodiment 2 to the UE or for each bearer of the UE and indicates a DAPS handover method (or through an indicator) to the LTE or NR PDCP layer connected to the UM DRB or if an upper layer (e.g., an RRC layer) of the UE indicates the DAPS handover method (or through an indicator) to the LTE or NR PDCP layer connected to the UM DRB, the LTE or NR PDCP layer connected to the UM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS.

When the DAPS handover method is indicated as described above, the source BS may transmit a handover command message to the UE with respect to the data of an indicated bearer and may immediately start DL or UL data forwarding to the target BS. Thus, when the UE transmits the PDCP status report (e.g., an indication of whether DL data has been successfully received) to the source BS with respect to the DL data, unnecessary data forwarding may be prevented.

If the RRC message (e.g., a handover message) received by the UE from the BS indicates the handover method corresponding to Embodiment 2 (DAPS handover method) to the UE or for each bearer of the UE, sets the DAPS handover method (or through an indicator) to the LTE or NR PDCP layer connected to the UM DRB, and a first condition as described above is satisfied, and thus a lower layer or an upper layer transmits an indicator (e.g., an indicator indicating UL data transmission switching) to the PDCP layer and the PDCP layer receives the indicator, or if the first condition is satisfied, and thus a lower layer (e.g., MAC layer) or an upper layer (e.g., RRC layer when the first timer is stopped) transmits the indicator to the PDCP layer and the PDCP layer receives the indicator, then the LTE or NR PDCP layer connected to the UM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS. When the LTE or NR PDCP layer switches UL data transmission, the LTE or NR PDCP layer may transmit a PDCP status report (or new PDCP control data) to the source BS or the target BS, and thus may indicate, to the source BS or the target BS, that a LTE or NR PDCP layer corresponding to the bearer of the UE has switched UL data transmission. Because the source BS or the target BS may identify a time point at which the UE satisfies the first condition, the source BS or the target BS may manage transmission resources of the UE by reflecting the identified time point in scheduling.

If the RRC message (e.g., a handover message) received by the UE from the BS indicates the handover method corresponding to Embodiment 2 (DAPS handover method) to the UE or for each bearer of the UE, sets the DAPS handover method (or through an indicator) to the LTE or NR PDCP layer connected to the UM DRB, and a second condition as described above is satisfied, and thus a lower layer (e.g., MAC layer) or an upper layer (e.g., RRC layer) transmits an indicator (e.g., an indicator indicating that connection to the source BS is to be released) to the PDCP layer and the PDCP layer receives the indicator or if the second condition proposed in the disclosure is satisfied, and thus a lower layer (e.g., MAC layer) or an upper layer (e.g., RRC layer) transmits an indicator to the PDCP layer and the PDCP layer receives the indicator, then the LTE or NR PDCP layer connected to the UM DRB may trigger and configure the PDCP status report and may transmit the PDCP status report to the target BS or the source BS. When the LTE or NR PDCP layer releases the source BS, the LTE or NR PDCP layer may transmit a PDCP status report (or new PDCP control data) to the source BS or the target BS, and thus may indicate, to the source BS or the target BS, that the UE has released connection to the source BS. Because the source BS or the target BS may identify a time point at which the UE satisfies the second condition, the source BS or the target BS may manage transmission resources of the UE by reflecting the identified time point in scheduling.

When the LTE or NR PDCP layer connected to the AM DRB or the LTE or NR PDCP layer connected to the UM DRB has performed triggering according to a triggering condition of the PDCP status report proposed in the disclosure, the PDCP status report may be configured as follows.

If the PDCP status report has been triggered in the NR PDCP layer connected to the AM DRB or the UM DRB or if the PDCP status report has been triggered, the PDCP status report may be configured as follows.

A first missing COUNT (FMC) value of the PDCP status report is set as a value of an RX_DELIV variable (COUNT value of first data not transmitted to an upper layer).

If the RX_DELIV value (COUNT value of first data not transmitted to an upper layer) is less than an RX NEXT value (COUNT value of data expected to be received next time), i) a length of a bitmap field may be set as a length corresponding to a multiple of 8 including from a COUNT value not including first lost PDCP SDU to a COUNT value of out-of-sequence last data, or ii) a length of a bitmap field may be set as a length from the COUNT value not including the first lost PDCP SDU to a COUNT value of PDCP SDU enabling the size of PDCP control data (PDCP status report) to be 9000 bytes. A length of a bitmap field may be set as a length corresponding to a case that is first satisfied from among the above-described two cases.

When PDCP SDUs corresponding to the bitmap field have not been successfully received or when header decompression has failed, a bitmap field corresponding to the PDCP SDU may be set to be 0.

When PDCP SDUs corresponding to the bitmap field have been successfully received, a bitmap field corresponding to the PDCP SDU may be set to be 1.

When the PDCP status report configured as such is transmitted to the lower layer, the PDCP status report may be transmitted as a first PDCP PDU of a transmit PDCP layer to the lower layer. That is, when a highest priority is given to the PDCP status report and the PDCP status report is generated, the PDCP status report may be first delivered to the lower layer and thus transmission is made fast.

If the PDCP status report has been triggered in the LTE PDCP layer connected to the AM DRB or if the PDCP status report has been triggered, when data is received due to re-establishment of the lower layer (e.g., RLC layer), the data may be first processed and then the PDCP status report may be configured as follows.

A First Missing PDCP Sequence Number (FMS) value of the PDCP status report may be set as a value obtained by adding 1 to an SN of the first lost PDCP SDU, a PDCP SN of the first data not delivered to the upper layer, or a PDCP SN of data last delivered to the upper layer.

If one or more out-of-sequence PDCP SDUs are stored in a buffer, i) a length of a bitmap field may be set as a length corresponding to a multiple of 8 including from the PDCP SN not including the first lost PDCP SDU to the PDCP SN of the out-of-sequence last data, or ii) a length of a bitmap field may be set may be set as a length from the PDCP SN not including the first lost PDCP SDU to the PDCP SN of PDCP SDU allowing the size of PDCP control data (PDCP status report) to be 8188 bytes. A length of a bitmap field may be set may be set as a length corresponding to a case that is first satisfied from among the above-described two cases.

When PDCP SDUs corresponding to the bitmap field have not been successfully received or when header decompression has failed, a bitmap field corresponding to the PDCP SDU may be set to be 0.

When PDCP SDUs corresponding to the bitmap field have been successfully received, a bitmap field corresponding to the PDCP SDU may be set to be 1.

When the PDCP status report configured as such is transmitted to the lower layer, the PDCP status report may be transmitted as a first PDCP PDU of a transmit PDCP layer to the lower layer. That is, when a highest priority is given to the PDCP status report and the PDCP status report is generated, the PDCP status report may be first delivered to the lower layer and thus transmission is made fast.

If the PDCP status report has been triggered in the LTE PDCP layer connected to the UM DRB or if the PDCP status report has been triggered, when data is received due to re-establishment of the lower layer, the data may be first processed and then the PDCP status report may be configured as follows.

An FMS value of the PDCP status report or a new field value is set as a value obtained by adding 1 to the SN of the first lost PDCP SDU, the PDCP SN of the first data not delivered to the upper layer, or the PDCP SN of data last delivered to the upper layer.

If one or more out-of-sequence PDCP SDUs are stored in a buffer, i) a length of a bitmap field may be set as a length corresponding to a multiple of 8 including from the PDCP SN not including the first lost PDCP SDU, first PDCP SDU not delivered to the upper layer, or next PDCP SDU of the data transmitted to the upper layer lastly to the PDCP SN of the out-of-sequence last data, or ii) a length of a bitmap field may be set as a length from the PDCP SN not including the first lost PDCP SDU to the PDCP SN of PDCP SDU enabling the size of PDCP control data (PDCP status report) to be 8188 bytes. A length of a bitmap field may be set as a length corresponding to a case that is first satisfied from among the above-described two cases.

When PDCP SDUs corresponding to the bitmap field have not been successfully received or when header decompression has failed, a bitmap field corresponding to the PDCP SDU may be set to be 0.

When a PDCP SDU corresponding to the bitmap field have been successfully received, a bitmap field corresponding to the PDCP SDU may be set to be 1.

When the configured PDCP status report is transmitted to the lower layer, the PDCP status report may be transmitted as a first PDCP PDU of a transmit PDCP layer to the lower layer. That is, when a highest priority is given to the PDCP status report and the PDCP status report is generated, the PDCP status report may be first delivered to the lower layer and thus transmission is made fast.

As another method, when data is received due to re-establishment of the lower layer (e.g., RLC layer), the data may be first processed and then the PDCP status report may be configured as follows.

An FMS value of the PDCP status report, a Next Received PDCP Sequence Number (NRS) field value, or a new field value may be set as a PDCP SN value expected to be received next time.

If one or more out-of-sequence PDCP SDUs are stored in a buffer, i) a length of a bitmap field may be set as a length corresponding to a multiple of 8 including from a PDCP SN value not including a PDCP SDU expected to be received next time to a PDCP SN value of data delivered to the lower layer lastly, next data, or first data not delivered to the upper layer in descending order, or ii) a length of a bitmap field may be set as a length from the PDCP SN value not including the PDCP SDU expected to be received next time to the PDCP SN value of the PDCP SDU enabling the size of PDCP control data (e.g., PDCP status report) to be 8188 bytes in descending order. A length of a bitmap field may be set as a length corresponding to a case that is first satisfied from among the above-described two cases.

When PDCP SDUs corresponding to the bitmap field have not been successfully received or when header decompression has failed, a bitmap field corresponding to the PDCP SDU may be set to be 0.

When PDCP SDUs corresponding to the bitmap field have been successfully received, a bitmap field corresponding to the PDCP SDU may be set to be 1.

When the configured PDCP status report is transmitted to the lower layer, the PDCP status report may be transmitted as a first PDCP PDU of a transmit PDCP layer to the lower layer. That is, when a highest priority is given to the PDCP status report and the PDCP status report is generated, the PDCP status report may be first delivered to the lower layer and thus transmission is made fast.

If the NR PDCP layer connected to the UM DRB or the AM DRB receives the PDCP status report generated as described above, when the NR PDCP layer has acknowledged successful delivery of each data having a bitmap field set to be 1 or having a smaller COUNT value than the FMC field, the NR PDCP layer may discard the data.

If the LTE PDCP layer connected to the AM DRB receives the PDCP status report generated as described above, when the LTE PDCP layer has acknowledged successful delivery of each data having a bitmap field set to be 1 or having a smaller COUNT value than the FMS field, the LTE PDCP layer may discard the data.

If the LTE PDCP layer connected to the UM DRB receives the PDCP status report generated as described above, when the LTE PDCP layer has acknowledged successful delivery of each data having a bitmap field set to be 1 or having a smaller COUNT value than the FMS field, the NRS field, or a new field, the LTE PDCP layer may discard the data.

If the PDCP layer has received the PDCP status report as described above and the data for which successful delivery is acknowledged in the PDCP status report has been delivered to the lower layer (e.g., an RLC layer), the PDCP layer may transmit, to the lower layer, an indication to discard the data, and the lower layer operates as follows in response to the indication.

When an RLC layer has received, from the LTE or NR PDCP layer, an indication to discard data (e.g., PDCP user data), and the RLC layer is an LTE RLC layer, the LTE RLC layer discards user data (e.g., PDCP PDU or PDCP Data PDU or RLC SDU) received from an upper layer (e.g., PDCP layer), if a portion of the user data has not yet been mapped with RLC user data (e.g., RLC data PDU) or has not been generated as RLC user data. When indicated from upper layer (i.e. PDCP) to discard a particular RLC SDU, the transmitting side of an AM RLC entity or the transmitting UM RLC entity shall discard the indicated RLC SDU if no segment of the RLC SDU has been mapped to an RLC data PDU yet. Therefore, if a portion of the user data has already been mapped with the RLC user data or has been generated as the RLC user data, the LTE RLC layer may not discard the user data and may perform data transmission to the source BS.

When an RLC layer has received, from the LTE or NR PDCP layer, an indication to discard data (e.g., PDCP user data), and the RLC layer is an NR RLC layer, the NR RLC layer discards user data (e.g., PDCP PDU or PDCP Data PDU or RLC SDU) received from an upper layer (e.g., PDCP layer) if the user data or a portion of the user data has not been delivered to a lower layer or has not been transmitted to the lower layer. When indicated from upper layer (i.e. PDCP) to discard a particular RLC SDU, the transmitting side of an AM RLC entity or the transmitting UM RLC entity shall discard the indicated RLC SDU, if neither the RLC SDU nor a segment thereof has been submitted to the lower layers. Therefore, if the user data or a portion of the user data has been delivered to a lower layer or has been transmitted to the lower layer, the NR RLC layer may not discard the user data and may perform data transmission to the source BS. Accordingly, the NR RLC layer, in contrast with the LTE RLC layer, may discard the user data if the user data has not been delivered to the lower layer even when the user data has been generated as the RLC user data, and thus may discard more data and may more efficiently prevent unnecessary data transmission.

FIG. 1K illustrates a UE according to an embodiment.

Referring to FIG. 1K, the UE includes a radio frequency (RF) processor 1 k-10, a baseband processor 1 k-20, a storage 1 k-30, and a controller 1 k-40. However, the components included in the UE are not limited thereto.

The RF processor 1 k-10 may perform functions for transmitting and receiving a signal via a radio channel, such as a band conversion, amplification, etc., of the signal. The RF processor 1 k-10 may up-convert a baseband signal provided from the baseband processor 1 k-20 to an RF band signal and transmit the RF band signal through an antenna, and down-convert an RF band signal received through an antenna to a baseband signal. The RF processor 1 k-10 may include a transmit filter, a receive filter, an amplifier, a mixer, an oscillator, a digital-to-analog converter (DAC), and an analog-to-digital converter (ADC).

Although only a single antenna is illustrated in FIG. 1K, the UE may include multiple antennas. The RF processor 1 k-10 may include a plurality of RF chains. The RF processor 1 k-10 may perform beamforming. For the beamforming, the RF processor 1 k-10 may adjust respective phases and amplitudes of signals transmitted or received through multiple antennas or antenna elements. The RF processor 1 k-10 may perform a MIMO operation and may receive several layers in the MIMO operation. The RF processor 1 k-10 may perform received beam sweeping by appropriately configuring multiple antennas or antenna elements, or may adjust a direction and a beam width of the received beam such that the received beam coordinates with a transmit beam, under the control of the controller 1 k-40.

The baseband processor 1 k-20 may perform conversion between a baseband signal and a bitstream, based on physical layer specifications of a system. For data transmission, the baseband processor 1 k-20 may generate complex symbols by encoding and modulating a transmit bitstream. For data reception, the baseband processor 1 k-20 may reconstruct a received bitstream by demodulating and decoding a baseband signal provided by the RF processor 1 k-10. According to an OFDM scheme, for data transmission, the baseband processor 1 k-20 may generate complex symbols by encoding and modulating a transmit bitstream, map the complex symbols to subcarriers, and then configure OFDM symbols by performing inverse fast Fourier transform (IFFT) and cyclic prefix (CP) insertion. For data reception, the baseband processor 1 k-20 may split a baseband signal provided from the RF processor 1 k-10, in OFDM symbol units, reconstruct signals mapped to subcarriers by performing fast Fourier transformation (FFT), and then reconstruct a received bitstream by demodulating and decoding the signals.

The baseband processor 1 k-20 and the RF processor 1 k-10 transmit and receive signals as described above. Accordingly, each of the baseband processor 1 k-20 and the RF processor 1 k-10 may also be called a transmitter, a receiver, a transceiver, or a communicator.

At least one of the baseband processor 1 k-20 or the RF processor 1 k-10 may include multiple communication modules to support multiple different radio access technologies. At least one of the baseband processor 1 k-20 or the RF processor 1 k-10 may include multiple different communication modules to process signals of different frequency bands. For example, the different radio access technologies may include an LTE network, an NR network, etc. The different frequency bands may include a super high frequency (SHF) (e.g., 2.5 GHz and 5 GHz) band and a millimeter wave (mmWave) (e.g., 60 GHz) band.

The storage 1 k-30 stores data for operations of the UE, e.g., basic programs, application programs, and configuration information. The storage 1 k-30 provides the stored data upon request by the controller 1 k-40.

The controller 1 k-40 controls all operations of the UE. The controller 1 k-40 may transmit and receive signals through the baseband processor 1 k-20 and the RF processor 1 k-10. The controller 1 k-40 writes and reads data to and from the storage 1 k-30. To this end, the controller 1 k-40 may include at least one processor, i.e., a multi-connectivity processor 1 k-42. For example, the controller 1 k-40 may include a communication processor (CP) performing control for communication, and an application processor (AP) controlling an upper layer, such as an application program.

FIG. 1L illustrates a BS for use in a wireless communication system, according to an embodiment.

Referring to FIG. 1L, the BS includes an RF processor 1 l-10, a baseband processor 1 l-20, a communicator 1 l-30, a storage 1 l-40, and a controller 1 l-50. However, the components included in the BS are not limited thereto.

The RF processor 1 l-10 may perform functions for transmitting and receiving a signal via a radio channel, such as a band conversion, amplification, and the like of the signal. The RF processor 1 l-10 may up-convert a baseband signal provided from the baseband processor 1 l-20, to an RF band signal and transmit the RF band signal through an antenna, and down-convert an RF band signal received through an antenna, to a baseband signal. The RF processor 1 l-10 may include a transmit filter, a receive filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc.

Although only a single antenna is illustrated in FIG. 1L, the BS may include multiple antennas. The RF processor 1 l-10 may include a plurality of RF chains. The RF processor 1 l-10 may perform beamforming. For beamforming, the RF processor 1 l-10 may adjust phases and amplitudes of signals transmitted or received through multiple antennas or antenna elements. The RF processor 1 l-10 may perform a DL MIMO operation by transmitting at least one layer.

The baseband processor 1 l-20 may perform conversion between a baseband signal and a bitstream, based on physical layer specifications of a first radio access technology. For data transmission, the baseband processor 1 l-20 may generate complex symbols by encoding and modulating a transmit bitstream. For data reception, the baseband processor 1 l-20 may reconstruct a received bitstream by demodulating and decoding a baseband signal provided by the RF processor 1 l-10. According to an OFDM scheme, for data transmission, the baseband processor 1 l-20 may generate complex symbols by encoding and modulating a transmit bitstream, map the complex symbols to subcarriers, and then configure OFDM symbols by performing IFFT and CP insertion. For data reception, the baseband processor 1 l-20 may split a baseband signal provided from the RF processor 1 l-10, in OFDM symbol units, reconstruct signals mapped to subcarriers by performing FFT, and then reconstruct a received bitstream by demodulating and decoding the signals. The baseband processor 1 l-20 and the RF processor 1 l-10 transmit and receive signals as described above. Accordingly, each of the baseband processor 1 l-20 and the RF processor 1 l-10 may also be called a transmitter, a receiver, a transceiver, a communicator, or a wireless communicator.

The communicator 1 l-30 may provide an interface for communicating with other nodes in a network.

The storage 1 l-40 stores data for operations of the BS, e.g., basic programs, application programs, and configuration information. In particular, the storage 1 l-40 may store information about bearers allocated for a connected UE, a measurement report transmitted from the connected UE, etc. The storage 1 l-40 may store criteria information used to determine whether to provide or release multi-connectivity to or from the UE. The storage 1 l-40 provides the stored data upon request by the controller 1 l-50.

The controller 1 l-50 controls all operations of the BS. The controller 1 l-50 may transmit and receive signals through the baseband processor 1 l-20 and the RF processor 1 l-10 or through the communicator 1 l-30. The controller 1 l-50 writes and reads data to and from the storage 1 l-40. To this end, the controller 1 l-50 includes at least one processor, i.e., a multi-connectivity processor 1 l-40.

As described above, are various efficient handover methods are provided for preventing occurrence of a data interruption time due to handover when the handover is performed in a wireless communication system, such that a service without data interruption may be supported.

While the disclosure has been particularly shown and described with reference to certain embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A method performed by a user equipment (UE) in a wireless communication system, the method comprising: receiving, from a source cell, a radio resource control (RRC) reconfiguration message; configuring a medium access control (MAC) entity for a target cell in response to the RRC reconfiguration message configuring a bearer to a dual active protocol stack (DAPS) handover; performing, by the MAC entity, a random access procedure on the target cell for the DAPS handover; transmitting, from the MAC entity, to an upper layer, an indication of completion of the random access procedure, in response to completing the random access procedure to the target cell; and switching, by a packet data convergence protocol (PDCP) entity, uplink data for the bearer, in response to receiving a request for uplink data switching from the upper layer based on the indication.
 2. The method of claim 1, wherein switching the uplink data comprises: transmitting the uplink data only to the target cell when resources for transmitting the uplink data to the target cell and resources for transmitting the uplink data to the source cell overlap.
 3. The method of claim 1, wherein switching the uplink data comprises: performing transmission or retransmission, from the PDCP entity, to a radio link control (RLC) entity associated with the target cell, the uplink data for which successful delivery is not confirmed by an RLC entity associated with the source cell, when the bearer is the an acknowledged mode (AM) data radio bearer (DRB).
 4. The method of claim 3, wherein the uplink data is transmitted or retransmitted in an ascending order of count values from a first PDCP service data unit (SDU) for which the successful delivery is not confirmed by the RLC entity associated with the source cell, and wherein the count values are associated to PDCP SDU prior to the uplink data switching.
 5. The method of claim 4, wherein performing transmission or retransmission of the uplink data further comprises: performing, by the PDCP entity, header compression on the PDCP SDU; performing, by the PDCP entity, integrity protection and ciphering on the PDCP SDU; and transmitting, from the PDCP entity to the RLC entity associated with the target cell, a resulting PDCP data PDU.
 6. The method of claim 1, wherein switching the uplink data comprises: transmitting, from the PDCP entity, to a radio link control (RLC) entity associated with the target cell, the uplink data which has been processed by the PDCP entity but has not yet been submitted to a lower layer, when the bearer is the unacknowledged mode (UM) data radio bearer (DRB).
 7. A user equipment (UE), comprising: a transceiver; and a processor coupled with the transceiver and configured to: control the transceiver to receive, from a source cell, a radio resource control (RRC) reconfiguration message, configure a medium access control (MAC) entity for a target cell, in response to the RRC reconfiguration message configuring a bearer to a dual active protocol stack (DAPS) handover, control, the MAC entity, to perform a random access procedure on the target cell for the DAPS handover, control the transceiver to transmit, from the MAC entity, to an upper layer, an indication of completion of the random access procedure, in response to completing the random access procedure to the target cell, and control, a packet data convergence protocol (PDCP) entity, to switch uplink data for the bearer, in response to receiving a request for uplink data switching from the upper layer based on the indication.
 8. The UE of claim 7, wherein the processor is further configured to control the transceiver to transmit the uplink data only to the target cell, when resources for transmitting the uplink data to the target cell and resources for transmitting the uplink data to the source cell overlap.
 9. The UE of claim 7, wherein the processor is further configured to control the PDCP entity to perform transmission or retransmission the uplink data for which successful delivery is not confirmed by a radio link control (RLC) entity associated with the source cell to an RLC entity associated with the target cell, when the bearer is the an acknowledged mode (AM) data radio bearer (DRB).
 10. The UE of claim 9, wherein the uplink data is transmitted or retransmitted in an ascending order of count values from a first PDCP service data unit (SDU) for which the successful delivery is not confirmed by the RLC entity associated with the source cell, and wherein the count values are associated to PDCP SDU prior to the uplink data switching.
 11. The UE of claim 10, wherein the processor is further configured to: control the PDCP entity to perform header compression on the PDCP SDU, control the PDCP entity to perform integrity protection and ciphering on the PDCP SDU by using the count values, and control the PDCP entity to transmit, to the RLC entity associated with the target cell, a resulting PDCP data PDU.
 12. The UE of claim 7, wherein the processor is further configured to control the PDCP entity to transmit, to a radio link control (RLC) entity associated with the target cell, the uplink data that has been processed by the PDCP entity but has not yet been submitted to a lower layer, when the bearer is the unacknowledged mode (UM) data radio bearer (DRB). 